Rules for being a date at a wedding?

My boyfriend of about 4 months just invited me to his friend's wedding (whom I've never met). He didn't mention that there would be anyone I know there, either. I've never been anyone's date to a wedding, are there any special rules about it? Do I bring a gift? Is there any specific ways I need to act around the bride/groom? Or any other faux pas I don't know about? Any advice is much appreciated :)