It sounds like the tractor won't turn over at all rather than cranking and not starting. Correct? Do all other lights light up (dash, headlamps)?
If so, first check the battery connections...especially the ground. Try to start.
Next, attach a battery jumper cable to the negative post (not the ground battery clamp) and to the engine. Leave the positive unconnected. Try to start.
Then attach the positive to the battery positive post (not the clamp again) and then the other end to the starter positive post where it connects from the solenoid (if there is a solenoid). You may want to disconnect the jumper ground (negative) first since there likely will be sparks. Let the sparks happen at the negative battery post. If the starter whirls then it is OK.
Finally, attach the positive to the solenoid positive input post (if the starter has an external solenoid...or one at all). The starter should engage and turn over the tractor. Make sure it is out of gear!!
These troubleshooting trials will get you started.