OK, so I launch FO3 fine, but every time I start a new game, it crashes, so I can't play! I don't know how to set it on compatibility mode however. Help me out please.

Sorry, can't do anything at all without all of the missing data you didn't include!

We need more information before we can help you. Please post a Dxdiag in spoiler tags by following the instructions on http://www.gamesas.com/topic/1122570-how-to-dxdiag/. If you're using any mods in your game, you need to post a load order as well.

To set compatibility mode, assuming that you're running under Windows 7 or 8, locate your Fallout3.exe file. Right-click it, select Properties, then click the Compatibility tab. Check the box labeled "Run in compatibility mode ...", select the mode you want from the drop-down, then click Apply.