2006 Nissan Pathfinder

My car was having cranking problems a couple of weeks ago, then finally it stopped cranking last Thursday. My father in law changed the starter because we took it to the auto parts store and had it checked after making sure the battery was good. The starter tested not good there. We bought a new starter and installed with no luck. My father in law thinks its a fuse but doesn't know for sure. I can't afford to send to the shop. Please help!
April 9, 2013.

Have a helper turn the key to the start position while you check for power to the smaller wire on the starter. You should have 12v. If you do, chances are you got a bad starter. If there is no power, you need to check the starter relay to see if there is power to it.