Go to Apple's online account creator from from the iTunes Store or open the iTunes program, click "Store" at the top of the screen, and click "Create New Account."
Fill out the information required by giving your email address, name, and a password of your choosing. Be sure to write down your password so you don't forget it!
Click "Continue" and your account is officially created. When you download and install iTunes, you'll need to input your email as your ID, so your store account will be tied with the iTunes software on your computer.