Which ad extenstion should be good for a forum?

[batman]

Starting and running a forum is actually a lot of work. The primary focus for you it has to be on getting traffic from whenever you can. As you said you are new to any kind of site management. So, I will recommend that first install your forum in a localhost and get a grip of the script and other what not's. There are lot of works which needs attention apart just from installing it.

Once you become comfortable with the script. You will on your own understand where to put ads and all. As I said your primary focus has to be getting traffic first and understanding how all of this works.