1） Start with plain water. Either go by fountains where you can drink from them, or carry some on a water belt. Then gradually over a period of weeks and months experiment with sports drinks and gels or bars.
2） There really isn't one correct answer for what to eat during the race. You need to try different things on training runs to find what works best for you.
3） When you find your "best mix", stick with it for your training runs and don't change when it comes to the race.