As I said make sure the battery is ok, put a digi multimetre on the battery as you try to start, if no drain when trying to start, put a 50 coin across the power in and power out of the solenoid, this will take the starter solenoid feed out of the circuit, see if it then spins(it won't turn engine over as it is not engaged), if it does you have a solenoid feed problem as this shows the motor part works.
You could always put a jumper cable directly onto solenoid carefully to try as well, or take off the solenoid feed and just put a live from the main battery input on it with a little bit of wire.
If you are getting serious volt drops ie 6/7/8 volts upon trying starter you have either a massive electrical resistance(very unlikely) or your battery is no good.
tabetha