Ok in this case I'm going to be assuming that you are totally new and have no idea what you're what your doing.
1) Open your web browser eg. Firefox, Google Chrome, Internet Explorer
2)Into the search bar at the top Type www.runescape.com this will bring you to the Runescape homepage
3) Click on the button that says "Free Users". This will take you to a screen where if you ahve the latest version of Java it will download the Runescape Cache if you download at about 200kbs (pretty average speed) this will take about 5-10 minutes.
4) Now there should be a small menu click on the button that says "Create Account"
5) Click next and enter your information, choose a name and password.
6) Now your done! Just log in with your new User name and password and get ready to enter the tutorial island!