You'll want to read the information about the kit you are ordering. Many times the kits come with yardage, not pre-cut shapes. There are some quilt kits that have the shapes already cut out, but I don't see that terribly often. That's how I first learned. I worked on a JoAnn Block-of-the-Month kit, where the exact shapes are laser cut.
Working with a rotary cutter is definitely a learned skill. Some kits come with a little extra yardage. You'd just have to ask the place where you're ordering it if that's the case. Either way, though, many times you can order extra of a particular included fabric if you make a boo-boo and cut a shape the wrong way.
Good luck & have fun!