go to the tab on the right, look for the cloth-looking thing with the red and white checkers. press it.
where it asks for Type: None, put Type: Image or Movie, then by image, press OPEN and find your texture. press it, open it.
once you have your texture, scroll down a bit to where it says "mapping" and change the projection from flat to whatever type you have,
if your shape is organic, then just put whatever its most similar too, press F12 to render, and it should show the texture.