You never want to approach these forums as just a place to get backlinks though, go in post questions you have and answer question you can in a respectful and helpful manor. Read all the rules of each forum. They do vary as to what you can and can not do but most will allow you to add a signature with the links to your own websites. Some wait until you have a few posts under your belt, usually 10 or so just to be sure your not spamming.
As well as backlinks, another benefit to the forums is that if you have good answers to other posters questions you will build up a good reputation in each forum and you will gain traffic just because people trust you and begin to follow the links in your sig to your offers.
Most forums will, however, not allow you to post your links in the content of the post this should be done in your signature and on your own profile page. Posting links and blatantly promoting your offers while offering no real value to the forum is a quick way to get yourself banned. Most importantly, have fun they can be addicting after awhile but the good thing is that your backlink efforts will pay off in the long run!