Set up your profile
When you find a community and forum that you like and plan on staying with for a while it is a good idea to set up and complete your profile. You access your profile by clicking 'My Profile' located at the top of the forum. This area is not much different than any other form you have filled out online or in real life except that much of the information in here is optional.

Personal Info
Summary - All your settings from other tabs summarized.
Edit Your Details - Edit your login name, email address and password.
My Avatar - Select an avatar from those available or upload your own image. If you upload your own image, you can upload an image up to 250 x 250 pixels and 50 kilobytes. The forum will automatically scale the image for use in various locations.

Forum Settings
Look and Layout - Tweak a few forum appearance settings.
My Profile Info - Fill in personal information that you don't mind sharing publicly. This is also where you fill out your forum signature.
My Posts - List of your posts ordered by most recent. Handy for finding a post that you don't recall which board you posted it in.
My Subscribes - List of topics that you have subscribed to send you an email notification for all new responses.
My Favorites - List of topics that you have ticked as a favourite of yours so they are more easily identifiable in the forum list.
Remember to press the 'Save' button any time you make changes in your profile.

Reply to an existing post
So, you have been lurking around the forum for a while reading every post and getting to know everyone. You have just read someone's post and you have finally decided you are going to reply and contribute some information of your own. Great! This is done by pressing one of the 'reply' buttons.
Quick Reply - this brings up a small text area (without reloading the webpage) where you can quickly type a short reply. Press 'Submit' when you are done typing your response. If you have a change of heart you can press cancel and nothing gets posted regardless if you typed anything or not.
Reply - This is the standard method in which you should reply when you want to reply primarily to the most recent post. The reason for this is because the subject line of your reply will autofill with the subject of the post from which you are replying. In case you were wondering, yes, you can change the subject. OK, now you are looking at what is called a text editor. Think of it as a slimmed down version of a word processor (such as Microsoft Word) in which you can format your text, add links, images, smilies, etc. More on how to format your post later. For now, type your response and press the 'Preview' button at the bottom of the page if you wish to view your post prior to submitting it. When you are done editing and are satisfied your post is ready to contribute to the forum then press the 'Submit' button.
Quote - If you wish to quote someone's post whether in whole or in part you simple press the 'Quote' button located beside the 'Reply' button underneath each post. The is very useful if you wish to make light of or expand upon a prior post. Once you press the Quote button you are brought into the same editing environment that you use to edit or create any other post. At this point, you type your response directly to that quote. When you are done replying, press the 'Submit' button.
Reply Topic - This button functions the same as the 'Reply' button with the exception that your subject line will autofill with the subject of the original post (OP). This may come in handy if you replying to a long post whose subjects may have changed over time but you want to reply to the OP. When you are done replying, press the 'Submit' button.

Create a new post of your own
Now that you have responded to a few other posts, and feel like an accepted member of the community, you want to tell us a little bit about yourself. To do this, you should start your very own topic so we can properly respond and greet you. Navigate to our General board and click on it so that you can see the list of all of the existing posts. At the top of the list is a button labelled 'New Thread'. Click that button to start your very own topic. You are placed at the same text editor that you used earlier when replying to posts. Simply type your message telling us all about yourself and press 'Submit'. Now you have your own post topic in the list.

Format your post
Remember we said earlier that the text editor is very similar to a word processor? Well, as in any word processor you can format and arrange your text for emphasis here in the forum.
The first, basic thing you should know is that the forum uses what's called BBcode or Bulletin Board Code. BBcode uses 'tags' to 'mark up' your text. Each 'tag' has an opening and closing 'tag' denoted within brackets. An opening tag will look like [ ] and a closing tag looks like [/ ]. Notice the slash in the closing bracket is the only thing that is different. If you forget the slash, the forum will interpret your closing bracket as an opening bracket.
The easiest way to 'mark up' your text is usually to type it out first, then select it, and press the appropriate button at the top of the editor. For example, if you wanted to make 'some text' bold you would type it out 'some text', then select 'some text' with your mouse, and then press the 'B' icon in the toolbar located just above the Message area (where you are composing your message). You will now see: [b]some text[/b]. When you preview or submit your message it will be displayed as 'some text'. Alternatively, if you know them, you can type the tags manually right along with the rest of your text or you can use the buttons at the top of the editor to insert the tags the fill in your text.
The BBcode Wiki covers the basic mark up tags so let's move on to what the Wiki doesn't cover.

Lists
There are two types of lists: ordered and unordered. An ordered list is a numerical list like you might use to list the finishing order of a race. An unordered list is a list you might use to list groceries. There are two different tags needed to create a list.
First, you use a tag that creates the kind of list you want.
[ul] [/ul] = unordered list [ol] [/ol] = ordered list
Second, you use a list tag for each item in the list
[li] [/li] = each list item.
So, the code for an ordered list would look like this:
[ol] [li] 1st place[/li] [li] 2nd place[/li] [li] 3rd place[/li] [/ol]

Videos
Embedding a video in your post is a more recent addition to the forum and makes use of many video sharing websites on the web. This gives us the ability to show the video in our forum without having to host it ourselves or making you follow a link over some other website to see the video. Once you have a video that you want to post you need to find the unique identifier in the URL to that video.
It's difficult to provide detailed instructions or examples here, so we'll only cover the basics. Understand that each video sharing website uses a unique identifier for each video. For example, this video from YouTube whose URL is http://www.youtube.com/watch?v=bqJE5TH5jhc. You need to know the video's unique identifier (which differs depending on what site you visit); in our example, the unique identifier is bqJE5TH5jhc. Once you know what that unique identifier is you can embed that video in our forum by selecting the video hosting website from the 'video' drop down list at the top of the WYSIWIG editor pr typing the following:
[video type=] [/video]
If you were embedding a video from YouTube, for example, the bbcode would end up looking like this:
[video type=youtube] insert_unique_identifier_here [/video]
for example
[video type=youtube] bqJE5TH5jhc [/video]
It's a good idea to preview your post to make sure the video is visible before submitting your completed post.

Edit your own post
There will be times when you make a post and you don't catch a misspelled word, fix a broken link or something that will require you to edit you post. It happens to us all so the edit button is easily accessible. When you are looking at one of your own posts that you need to edit you will find the 'Edit' button at the bottom of your post. Simply press the edit button and you use the same editor you used to create your post populated with the actual post. Edit your post as you see fit and, when you are satisfied, press the 'Submit' button at the bottom of the page. All done!

Visibility and notification of your favorite posts
Now that you have settled into your new community and everyone has met you, you want to be able to keep up with your favorite posts without checking in every five minutes to see if something new has posted. There a couple of ways that you can do that.

Subscribe
At the top and the bottom of every thread you will find the 'Subscribe' button. Pressing this button subscribes you to that topic so that any responses to that topic will generate an email notification to your email address stating that a new post has been made along with a link directly to said post for your convenience. You will notice the button now reads 'Unsubscribe' indicating that you are subscribed and pressing the button again will remove your subscription to that thread. Remember that you can also manage your subscriptions in your profile.

Favourite
Another way of gaining visibility of threads that wish to keep an eye on is set them as your 'Favorite'. This will place an obvious indicator, such as a star, beside the topic name of the thread so that it will stand out in the forum list of topics. You will notice the button now reads 'Unfavorite' indicating that the post is already one of your favourites and pressing the button again will remove it from your favourites. This is a great way to get visibility on multiple topics that you wish to watch without getting spammed by email subscriptions. Remember that you can also manage your favourites in your profile.

Source: Joomla forum help page