if you are using internet explorer; then click on tools. then go down the list until you see "internet options". go through the tabs (it should be the first) until you see a blank white space. type in "www.runscape.com" (without the quotation marks.)
if you are using mozilla firefox; then go to tools, then click on options. click on the tab labeled "main" and you will see the address of your current home page. if you see "about.blank" then i guess that you start out blank. delete what ever your homepage is and then type in whatever you want it to be. (which i am guessing is runescape.)