The answer to the question lies in one word - DON'T. Why? Think about it. There are no re-shoots. If you mess up, it is recorded for all time. Then there is the little matter of two entire families relying on you to capture the memories of one of the most important days of their lives. You will also have to research the location and logistics of the shoot. Finally, even if you are prepared, you have to pray your equipment does not screw up. Still want to do it? Well, here are some things to keep in mind.
You only get one shot at this. And remember the preparations could have taken up to a year. The event will have cost money and time. Make sure it does not go to waste! You do not have to be the best photographer in the world, but you do have to be professional even if you do not have the credentials yet. This means that you should show up on time, be polite, be efficient, and carry out your task with a pleasant demeanor. Do not be unhappy on a happy day. It may not even be a bad move to take out indemnity insurance for any unfortunate occurrence just to protect you from any potential legal disasters that may arise.
The couple has a million other things to worry about. Your photos are something that will be with them for a long time to come so make sure they are well done. But be aware you are also taking pictures for their entire families. Keep in mind you may be called upon to put together albums for other people in the wedding.
Keep in mind that your preparation is a contributor to your performance or lack thereof.
Meet the people involved in the wedding
. Do a practice run. Visit the locations. Plan your special shots. Imagine what an idiot you would look like if you forgot the names of the bride and groom or if you got lost on the way to the wedding. Type up a list just to make sure.
This may seem obvious, but make sure you have the right equipment for the job.
If you are still deciding about that, then you are in big trouble. It is true that it is not about the camera but about the photographer. But even the most talented photographer will not be able to do much with a disposable camera. A minimum standard would be a 35mm SLR film camera or an 8-megapixel digital SLR. You should have lenses ranging from ultra wide to medium telephoto. One decent speedlight would be good too. The most important thing about your equipment is that you have to know it very well. Understand how it works and understand how to make it adjust to unexpected situations. Spare batteries and even a spare camera are both good ideas.
Hopefully, you'll get through your first wedding shoot okay and live to tell about it!