Wear clothing that you are comfortable in. Look nice of course, but try to be as comfortable as possible. Most weddings and receptions are an all day event. It's hard to have fun when you're uncomfortable.
Step2
Be friendly. Don't just talk to the people you know. Everyone here is a friend of a friend. Sitting alone in the corner is not the way to have fun.
Step3
At dinner enjoy your meal. Have a drink. Talk with the other people at your table. Stories about the bride and groom will involve everyone at the table and will almost always bring a lot of laughs.
Step4
Lastly and most important... DANCE, DANCE, DANCE! When the festivities get going, get out on the dance floor. It doesn't matter if you know how to or not. If there's nobody out there, be the first. Others will quickly join in. All the goofy dances are the most fun. Do the chicken dance! Do the hokey pokey! Sitting in your chair watching everyone else having fun is not the way to do it. So get out there!
Tips & Warnings
Weddings can be incredibly romantic. If you can bring a guest, a significant other is a great way to go.