Welcome to the world of knittiing!
Casting on too tightly is a common problem, especially for brand new knitters. You could get a set of needles a couple of sizes larger than the ones you will knit with and cast on with those, and then knit with the ones you will be knitting with. You could also take your knitting needles and put them together with the points going the same way. Cast on around both of them, and then pull one needle out before begin to knit.
Getting even tension is something that will come with time and practice.
If you haven't yet, take a look at the videos on this site. They're excellent!