Verifying is the easy part - you just enable:
$enable_dkim_verification = 1; # enable DKIM signatures verification
This is from the amavisd.conf-sample file, and indicated in the "A QUICK START TO DKIM VERIFICATION" portion of the amavisd release notes.
Setting up signing is not trivial, and can't be explained without a fair amount of learning on your part. There are a number of good threads in the amavisd-new mailing list archives (look for DKIM in the thread's subject). These are some very useful conversations between other users like yourself and amavis' author. I suggest spending time reading the threads (any since the release of amavis 2.6.0).
I'm happy to help, but we'll need to get to specific questions. I understand it is difficult to start when none of the terminology means anything to you. Perhaps you're not ready for this, and should first focus on learning the prerequisite terminology and knowledge referenced in the A QUICK START TO DKIM SIGNING (or VERIFICATION).
You can test the configuration by sending mail via the signed domain to: firstname.lastname@example.org