It's easy to get started helping out users on the forum: just browse the thread list for people looking for help. If you want to talk about giving support, or if you have suggestions or feedback, you can use the Contributors forum.
While that's really all you need to know, there are many things you can do to be able to provide more and better answers. Below are some tips to make you more effective.
As mentioned above, use the thread list to find users in need. Pay special attention to threads that have no replies or where the last poster wasn't another contributor (contributors have names marked in green). If you're on Mac or Linux, watch out for users on the same OS; they may have questions that Windows users wouldn't be able to answer.
Rather than trying to figure out the user's problem by yourself every time, first search to see if it's come up before. Try to use the Knowledge Base articles before anything else. These articles have been quality reviewed and have info for all supported systems. Remember, you're not only helping the person who's asking the question, you're helping people who read the thread in the future.
If the Knowledge Base doesn't have your answers, you can use any other source you want. Some good ones:
If you find yourself using external resources, consider adding their information to an existing article, writing a new article, or requesting that the article be written. Because this site is the first line of Firefox support and a Knowledge Base search is the first thing most users will do, having the info in a Knowledge Base article will let the user find the info more easily by themselves.
If you can't find previous cases of the issue happening, here are some tips on figuring out the problem yourself.
Again, if you find out the solution, consider adding it to the Knowledge Base.
The answers you give may not be correct, or the user may have some follow up questions for you. In either case, it's useful to both you and the user that your conversation continues. Consider checking "Send me an e-mail when someone replies" on threads you post in so you know about any replies. You may also find the list of threads you posted in useful.