If you want to perform a wedding, it's surprisingly easy to get ordained to do so. In most states, any recognized member of the clergy can perform a wedding ceremony, so all you need to do is get properly certified through a recognized church, and you can marry a couple.
Check with your local courthouse or registrar to find out if there's an easy way to get certified. In California, Massachusetts and Alaska, you can fill out a form at the courthouse that allows you to perform marriages within a 24-hour window. If you live in one of those states, you're all set.
If you live in a different state or want to perform more than one marriage, request ordination through the Universal Life Church. It's recognized by 49 of the 50 states. Visit one of the church's unofficial sites at ulc.net, and click on the "Becoming Ordained" tab.
Enter your information as requested in the spaces provided. Be sure to provide your correct name and email address, since that information will be used to put together your ordination.
Once the Universal Life Church has confirmed that you are a real person, it will confirm your ordination via email. The confirmation is free, though you can purchase a framed certificate and other documentation.