Hey I was wondering how do i put in a different pants and t-shirt etc. on instead of the defaults. Thanks

It depends on what character you are using. Some of the characters were created with the clothing already on. For those characters you cannot change the clothes. You can, however, change how they look through textures and bump maps. Some characters may have clothes attached that can be removed. For those characters you simple select the clothing and then press delete on your keyboard.