A wedding guest's only job is to show up at the ceremony and reception on time, smile and wish the happy couple well. You don't have to do anything else unless you've been specifically asked to participate in another way. However, there are a number of things you should never do at a wedding:
Don't change into casual clothing halfway through the reception. This goes for children too. Yes, sometimes it's hot and sticky at an outside venue, but at the end of the day, you and your family will show up in the background of all the photos looking dressed for a picnic instead of a wedding reception. It's unfair to the bride and groom who've paid for expensive professional photography and gorgeous décor.
Don't drink like you've never been to an open bar before. This is not spring break — it's the most important day of the bride and groom's life so far and you should behave appropriately.
Don't steal the microphone if you haven't been asked to make a formal toast. Don't make your spontaneous karaoke debut to the obligatory Michael Jackson mix. And don't jump into family pictures where you clearly don't belong. The bride and groom will invite you to toast if they want you to, they'd have asked you to sing if they wanted to hear you, and they really do want some fantastic family photos without goofy unexplainable friends in them.
Never assume you are invited to visit the bridal suite on the day of the wedding if you're not her mom or a member of the wedding party. Even if you are a good friend or family member, the space will start to get very tight (real or imagined) for the bride if too many people are there while she's getting ready. Just picture scenes from My Big Fat Greek Wedding if you don't understand. If the bride invites you, then you belong there.
Do not criticize the décor, food, bridesmaid dresses or anything else about the wedding while you are at the wedding. Somebody will overhear you and be offended and repeat your unkind remark to the couple or one of their parents at some point down the road. Avoid the embarrassment of having to explain and apologize.