I've done lots of marathons and have made a lot of mistakes over the years!
First of all - respect the distance. People will tell you the race starts at 20 miles and until you've done it, this doesn't make any sense. 26.2 miles can't be that different to the 20 or 22 you do in training, right? Wrong. A marathon is a 20 mile warm up and then a six mile race. The aim on the day is to get to 20 feeling relatively strong and running within yourself; and then dig in for the last six. As otherwise the wheels will fall off at 22/23 and it isn't pretty.
Fuelling is key - and practice in training with the gels you will be using on the day. Do not practice with SIS gels if on the day you plan to use the Hi5 gels they give out on the course. Horrendous explosions will ensue.
Training- follow a training plan and get your long runs in. The training won't go exactly according to plan, as life will get in the way, but you want to get to the start line uninjured, knowing you have those long runs in the bag. For a first time, follow a training plan based on time on feet rather than distance too, as if you doggedly try and eke out 20-22 mile runs and it's taking you 4+ hours, this is going to be setting you back rather than getting you fitter. Really, you don't want to be doing any long runs of more than 3h 30 in training as the risk of injury outweighs the training benefit.
Finally - and related to the last point - the main thing you need to do during training is avoid getting injured. Listen to your body. If you up the mileage too quickly and things start to hurt, ease off for a week. Many first time marathoners push through the pain, then get injured and turn up on the start line with a niggle that then ruins their race. In your first marathon, any time you run is a PB, so the most important thing is to be able to get there, and get round, in one piece.
Good luck and enjoy!