Put your tractor in neutral, and set the brakes. Turn your ignition key to the run position.
Make sure you touch the correct side of the solenoid. The correct side has the large cable going directly to the starter.
To eliminate the possibility of a bad tractor battery, use a set of booster cables attached to a good booster battery. Make sure you have a nice tight connection between the negative cable post on the booster battery and your tractor frame. Also make sure your positive post connection on your booster battery is nice and tight as well. Then touch the large terminal on your starter. You will probably see a nice size spark. Hold it tight on there, and it should crank. If all connections are clean and tight, this procedure should get your starter to engage and crank over your engine enough to start it.