When I first started doing the forward rolls on the ground many years ago, I was going all over the place, rolling in all kinds of different directions until I got a great tip from one of the brown belts:
The main thing to remember is to keep your arm extended in a circle, and don't let it collapse (this is a really important principle, not only for safe rolling but for the "circularity" of Aikido in general.)
Also remember that whatever direction your elbow is facing, that's the direction you'll be rolling - sounds simple and logical, but I've seen *many* beginners who (like me when I started out) end up rolling all over the place because they aren't always shown the proper way to do it.
Good luck with your Aikido training, let us know how it's going and be sure to ask questions - that's the way to learn
-Bryan