Before start this project, you might want to measure the size of your head. You can use with a measuring tape. Find out how many inches the circumference of your head is.
In order to get the rigth size of your beanie; make path by knitting a few stitches using your knitting needle and yarn. Check how many stitches you need to make up an inch, and then multiply that number by the inches of your head. Thats how easy it is to know how many stitches you need to make when you first cast on.
For example: If your head is about 22 inches and you need 2 stitches per inch, (depending on sizes of your yarn and needles) you should multiply 22 x 2 to get 44 stitches for your cast on.
God Luck with your knitting project!