Rigging ???

For my game I’ve built the armature structure. (When I press ctrl it crouches, when up arrow it walks,etc) Now to make it look good I need to attach a mesh(with clothing and all) which I downloaded from the internet.(since I’m not good at modeling) But when I do it using the arnature modifier it wont deform as the structure walks. It just moves with it. I really don’t know much about Blender. Can somebody please explain me how to attach a mesh to the armature.

Right click the mesh. Shift+right click the armature (in object mode), then CTRL + P.
then select either of the bottom five options, each doing what they say. You may need to weight paint. (google it)