When you have a dedicated visitor base with plenty of return visitors, you are ready to go. Anything that gives visitors the means to interact with others. This can be anything from forums, to voting booths, to live chat . I advise not to implement all the tools at once. Put one up, let it develop, then put another so it can draw from the previous.
Most people start with forums .
Generally, this is a great idea. If you're not sure how well a forum would go on your site, try putting up a voting booth first. Find out what percentage of your visitors are interested in offering an opinion. When you decide to put up forums, prepare to spend some time with them. Remember what I said earlier about nobody wanting to be part of a dead forum.