An over-tight mask is a common reason for leaking. The more it leaks, the tighter it gets adjusted...making the problem worse.
The mask strap should be sufficiently tight to hold the mask against the face without it slipping. That is all. Once submerged, water pressure will ensure the mask creates a seal against the face.
Some masks won't fit some faces. It's worth trying a few different masks, to ensure that this isn't the case with your current mask. Also, facial hair, or deep skin creases around the eyes or mouth can cause leaks.
If your mask is leaking underwater... have your buddy examine it. They will see where air bubbles are escaping from, which will indicate where the leak is.