Start by measuring the voltage to ground at both the battery
connection at the starter, and the solenoid "start" terminal while
(carefully) a friend tries to start the engine.  If both these are
roughly battery voltage, the problem is your starter/solenoid.  If
either one is low, then you have a wiring problem in that circuit.

Alex
(Just made it to Vegas from San Diego in 110-115F heat.  Not that I
don't have a huge list of things to fix, but I have to admit I felt
some pride rolling in past so many cars broken down on the side of the
road.)