Dressing the groom and groomsmen for a wedding can be the easiest, most difficult task you encounter. Easiest because there are strict guidelines and rules that are simple to follow. Most difficult because no one seems to know them or desire to follow them. And we weren't all born with Brad's fashion sense...
There are quite a few resources that will tell you how to dress. Here are two that are particularly good. One is a long, in-depth speech to wedding professionals from a male fashion expert that gives great photos as examples at The London Lounge.
Another at Put This On gives a more plain language speech about how to dress. I particularly love the last paragraph of his article
Weddings are a celebration of love. They’re also a very serious ceremony, one of the few we have left. We wear traditional clothing because it shows that we care about this commitment we’re making. It’s a commitment not just to your partner, but also to your family and community. Wearing traditional clothing demonstrates respect both for the person you’re about to marry and for the community of friends and family that will support you in marriage.
However, the bottom line is there are two main rules every man MUST follow (in my book):
1. NO TUX BEFORE 6 PM!
2. GET YOUR SUIT PROFESSIONALLY TAILORED!
If you can adhere to those guidelines (and take a quick glance at the advice of the experts) you're sure to look smashing on your wedding day.