The battery is fully charged and it wont start.

Measure the battery voltage first. It must be 12.6 volts. Turn on the head lights to insure some current is trying to flow, then use a test light to test for voltage on the battery cable clamps, not the battery posts they're attached to. Move the probes down the cables to the next accessible points where you can measure. Look for where the smaller black, negative wire bolts to the body, and see if there's a smaller red positive wire bolted to the under-hood fuse box. Be sure all those connections are clean and tight. You're looking for the first place you don't get the same voltage. That will be the point of the bad connection.