To start any knitting project you have to make the loops on the needle that you will then knit to form the fabric. This is known as casting on.
Performing a knit cast on is handy because you basically learn how to make the knit stitch at the same time, but the wrap cast on is also quick and easy, if not quite as firm. (I used the wrap cast on method on the swatch pictured.)
You might want to practice a bit before you make the final cast on for your knitting, making sure that all your loops are relatively even in tension.
When you're ready to begin the project fully, cast on 20 stitches in the method of your choice.