How do you put mods in fallout 3

If you are asking for an X360 version, it is not possible.
But for the PC if you go to where it is installed you will find a folder called Data, put your mods in there. for help loading mods, use Fallout Mod Manager, because certain mods require other mods to launch first.
E.G. A companion mod i made, called 4 New Companions, that had to be loaded after RRcompanionVault.esm