Sharon-

I would first ask a couple of questions:

1. How many users does your site have?  Do you expect it to grow much beyond that number?
2. Do you have any money to work with - would $50 to $100 break the bank?

As Sebastian said - there are three forum modules available for DNN.  They break down as follows (my opinion):

If you have a small number of users and the ability to troubleshoot and fix things when and if they break the best choice is the core forum module on codeplex - it's free.

If you have a larger number of users and the ability to troubleshoot and fix things YAF might be the way to go.

If you just want it to work day in and day out, don't really want to have to fix things, and can afford $50 - $100 then Active Forums is the way to go.  I think Active Forums probably has the best documentation - good enough that you could install and set it up without any problems.  Plus Active has more features that you can grow into down the road such as the forum viewer module and the ability for users to send a post in via email (mail connector).

I'd be glad to help if you need it - leazoninc at earthlink dot net