For anyone who isn't familiar with weddings and doesn't know what's expected or what to expect, here's a few rough guidelines. Obviously I don't know exactly what's planned so I'm keeping this as generic as possible and sticking to the things that some people will think are obvious.
1) Don't wear white. In this part of the world, that's the colour the bride wears, and she's supposed to stand out. Avoid anything pale enough to be mistaken for white, too.
2) The first part of the wedding is the ceremony where the couple say their vows. The officiant sometimes gives a sermon or advice as well. For this part all you have to do is sit in the audience and watch. Don't talk to people or use your phone.
2a) Avoid taking photos with the flash on, too. It's distracting, and if there's a professional photographer it might interfere with their shots.
3) After the ceremony is the reception. There'll probably be speeches from some friends and family. Sometimes people who haven't been asked to can say something themselves as well, but it shouldn't be very long.
4) There's usually alcohol at the reception. If you're drinking stick to your limits and don't make anyone - especially the couple - have to deal with very drunk people tripping over things, passing out, throwing up, etc.
5) If there's assigned seating (there might not be - I feel like that could be a little difficult here with people coming and going, and some weddings are just less formal) there's probably a reason you were put where you were. Talk to the people you're sitting with. The couple has reason to think you'll get on, or at least won't cause World War 3.
6) If you're going to hook up with someone, don't do it in a closet or bathroom or behind a curtain. At least go somewhere with a bed.
7) Keep things positive. You're there to celebrate people being in love. It's okay not to like all the details, but everyone doesn't need to hear about it, especially right then.
8) Wear something nice that you feel comfortable in. No ripped jeans, but you don't need to go full formal suit or ballgown. You don't even need to wear a dress if you don't like them. It's a fun excuse to dress up but you're not going to get kicked out if you have the wrong hemline or don't wear a tie. (Probably, at least. Typically that kind of strict pedantry doesn't come from people who send out essentially open invitations.)