Now days there is a ruler for everything. However, to get started with the basic bare bones you can get by with a 6 x 12 and a 6 x 24 inch ruler. I happen to like the Omni grid rulers, but there are many good ones out there. With those 2 rulers, you can cut strips, squares, even triangles, sort of.
If you want to get a little fancier, there are some great rulers at
www.guidelines4quilting.com/
It's complicated to explain how they work but it is worth checing them out. I bought 2 of their 6 x 12 rulers and a connector. Those 2 can be combined into all sorts of combinations. I like to use this set up for repetitive strips. They are fast becoming my favorites. I teach a beginner's class through adult education in my local high school and my students always want to borrow them in class.