That depends:(1) what time is the wedding?(2) how formal is the wedding? You can get some idea of that from the invitations--are they engraved on off-white paper, or are they cute/contemporary?(3) on your age and personal style.
For a daytime spring wedding, a flowered dress is always good. Or a really nice sweater and a skirt, if it's less formal. Personally, I'd stay away from black during the day.
If it's evening, dress more formally--like a party dress or a jacket/dress suit. Some people say no black, but in the evening people do wear black (esp. in the winter). Nothing too flashy or revealing, though.
Also, if it's a church wedding you should be more conservative.
These are just the things I see done in my area--I certainly don't know all of the rules! Have fun.
You're so right, Lariea!