You should get your head checked. Friends don't shoot friend's weddings. Relatives don't shoot relative's weddings. Friends don't shoot relative's weddings. Relative's don't shoot friend's weddings. I'm tempted to say if you haven't shot a wedding before you shouldn't shoot one now. In fact, I'll say it. Despite the obvious faulty logic—eventually no one would ever shoot weddings in the future if they can't shoot a "first one”—if you fail to take the advice that's appeared up to this point in my answer, you're headed for a potential disaster. A disaster that starts life-long feuds and creates ex-friends and disowned relatives, and may even end in court.
A bride that doesn't get what she expects from a wedding photographer, or worse still, fails to get any of the crucial moments captured forever, is not a thing you want to encounter in the wild (or in civilization, either). The more expensive and elaborate the wedding, the more guests, and the more upscale the location, the easier it will be to fail at something. But even if it's a dozen people gathered in the woods for five minutes, you're still likely to fail. Even pros screw up once in a while, which is why all smart wedding photographers carry insurance and hire assistants to do supplemental (backup) shooting. And, no, your D3300 with an 18-55mm kit lens isn't going to cut it (which you probably knew or you wouldn't have asked the question).
No, the correct thing to do when confronted by someone asking you to shoot their wedding is this: say "I'd be happy to bring my camera and do some candid shooting at the reception for you, but you should hire a professional photographer who knows what they're doing and will deliver everything you want." Then, when the day arrives, you should introduce yourself to the pro and say "look, my friend/relative/insane acquaintance/Bridezilla asked me to do some casual shooting for them, but I want you to know that if you think I'm in your way at any time, just ask and I'll defer 100% to you. I understand you have a job to do and I don't want to get in your way." And then do everything to stay out of the pro's way the rest of the day. Don't bogart their shots; find your own (where you won't be in the pro's frame).
Sure, you may win the lottery and get away with a wedding shoot that works and doesn't displease anyone. But the odds are considerably stacked against you.