Casting on too tight is common for new knitters. Try not to pull the yarn too tightly. You can also try casting on to a larger needle or two smaller needles held together.
Which cast on are you using? Some are better than others for learning to knit especially. If you used the backward loop I suggest you use another like the knitted on cast on. Backward loop is really only good for a short number of stitches and is often used when you need a cast on in the middle of your work.