RIFLE APPROACH 1) Find relevant forums 2) Find relevant threads in those forums 3) Use a relevant sig and create or respond to relevant discussions SPAM APPROACH 1) I wouldn't recommend it.
Earning from a forum is a challenge honestly and you shouldn't just pick a topic, and start a site and expect people to join. It takes a lot more than that. Forums takes patience, constant tweaking, and even lower CTR conversions. I find it best to have a good chunk of change and really invest into the community you are trying to build. Offer contests, unique threads and a friendly environment and the money will come after you have a functioning and decent community. The best thing to do is just have a google ad or two, affiliate offers, and premium memberships those always do the best for me.