I'm just starting with CJ and affiliate marketing in general. I want to advertise banners on forums (forums that I don't own) related to the product I'm trying to advertise. Is this allowed? Do I have to register it under my websites?
Register website there help to track sales from that site. You can advertise affiliate banners on other sites.(if that site allow)
You don't need to register the site under your websites section. You can just use SID section to track the sale.
All you have to do is cut and paste the code. CJ doesn't care. The Advertiser "might" care... depends on how big they are. If you are concerned you should just write them and tell them what you want to do. Banners are notoriously low converters. And if you think you can make money paying to advertise on Forums, and can get enough of a return on your investment then you should give it a try. If you have permission to advertise on a Forum you could just set that Forum up as a site in your CJ Admin Panel so you can track the impressions.