There are many different places that you can have forums hosted, and not all of them even cost money.

Since you sound rather new to forums, I would suggest starting on a free forum so that you can practice and learn the ropes.

Once you have a grip on what to do, then you can consider moving on to a pay host.

Starting a large,successful forum takes a lot of time,work, and effort.

Not even paid advertising campaigns are always the answer.

A few free forum hosts you could try out are forumotion.com, freeforums.org, and any number of other ones.

Best of luck to you!