Creating your account (for students under 9 I set this all up for them) is easy. You do not have to down load anything for scratch to work but you need a scratch account. To set up your account go to http://scratch.mit.edu and creative an account, ask your teacher first you may already have an account.
Having trouble or want to know more about your account watch the video:
Already a user of scratch then logon scratch.mit.edu