car will not turn over, will crank but no start. What do i check first.

Disconnect the glow plug on the left back side of engine. Then go get someone who knows that a relay IS a switch.If the battery was hooked up backwards then you more than likely blew the 'main' fuse(probably 80 or 100 amp) and quite likely some others and maybe burned some fusible link(s).Cut your losses and definitely get someone qualified (and I DO mean that in a nice way).