Bad ground?
Describe exactly what it does.
Examine carefully the ground cable, should go directly from the engine block to battery NEG
Examine the battery cable should go from starter relay to battery POS. A second large cable should go from the same post on the starter relay to the starter
Stick a screwdriver between the two posts on the starter. Make CERTAIN it is in park/ neutral IE "out of gear" and the starter should crank.
If it does not, all you have there is the two cables, the starter, and the battery.
Do you have a meter/ test lamp? You need a multimeter, and a bag of Radio Shack clip leads.