how to map an image to A SELECTED FACE?

how do I map an image to a selected face instead of mapping it to the whole object?
for example something like a logo…?

Well, a logo on a shirt would be best mapped to an object, like a decal. See Textures user manual for info on MapInput to decal/object.

To map an image to a face, select the face and unwrap it to a UV map, and then load that image in the UV/Image Editor, (see UM on Painting->UV)