I agree with everyone. A snaffle is definately the best way to go. Personally, I tend to start in an eggbut snaffle and then work from there. If you find your horse has trouble with steering, I'd move up to a D-ring or a full cheek. If she tends to fight the bit, try a rubber snaffle.
I am training a 4 year Arab stud and all he needs a full-cheek rubber snaffle to be happy.
Make sure whatever bit you use on her she likes. The more drool the better. I added the rubber for the stud to encourage him to give to the bit and it has worked wonders.