Instructions
Download and extract a mod you want to use from a reputable website.
Open your "Fallout 3" installation folder, which you specified while installing the game.
Double-click "data." Right-click the window and create a new folder, naming it whatever you want.
Drop and drag the files you downloaded into the new mod folder.
Double-click the "Fallout 3" icon on your desktop and wait for the window to appear.
Click "Data Files," which opens a dialog box. Click the check mark box beside your mod, and then click "OK."
Click "Play" to load the game with your mod enabled.