I know there is another thread about this, but I will write one out which is easier to use and understand.
Step 1. Download firefox. Get it from http://www.mozilla-europe.org/en/firefox/ (If you don't have it already).
Step 2. Once it's downloaded and installed, click Tools then click Add-ons. Click the left tab that says "Get Add-ons" and search for Adblock Plus, (or just Adblock if it doesn't find it) then add it. Here is a direct link to the Add-on if the method above doesn't work. https://addons.mozilla.org/en-US/firefox/addon/1865. After it's installed, restart Firefox.
Step 3. Click Tools then click Adblock Plus Preferences (Or click CTRL + SHIFT + E). When the Adblock screen comes up, click Filters then click Add filter subscription. Click the radio button next to EasyList (USA) then click the Subscribe button at the bottom. Wait for Adblock to update with this list then click OK.
Open the Adblock window and click Add Filter then write in "runescape.com##DIV#menu" (Without the quotation marks). This will block the bar at the top of the screen which will now actually give you full screen! (Thanks for this tip Harry)
Step 4. Go to runescape, go to any world and before logging in (You will notice that the add at the top is gone and the client is pushed up), click Graphics Options, then click "Resizeable". I also recommend doing High Detail if your computer can support it.
Step 5. Click outside the runescape client (Either left or right the bar thats ontop of the runescape client) then click F11. If that doesn't work, click the View tab on firefox then click Full Screen. You will notice that the small bar is still ontop, but other then that, you have runescape in full screen, without members! Press F11 to exit out of fullscreen.