Samantha -
Good advice so far; I'll just add one thing. You will eventually be buying several rulers. The teacher where we are taking our class recommends buying one brand of ruler and sticking with that brand. I think it's great advice, because you will grow accustomed to the markings and "layout" of the ruler. When you move from one ruler to another it is very handy to know what to look for, how to turn the ruler, etc.
I know a lot of people on here use the Omnigrid rulers, and they are great. We are using Creative Grids, and we love them. It will eventually come down to personal preference. Just look at several, and if you can get some pointers from the salesperson ask the pros and cons of the ruler line they are selling.
Enjoy your new hobby.
Darren & Kimberly