You really need a volt meter to accuratly trouble shoot your problem.
Start with cheching battery voltage. Should be in the 12.6V range.
If good there, follow the positive cable to the where ever it hooks to next (probabally a solenoid) and confirm the voltage there. If all that is good, now check the voltage to the smaller wire at the solenoid with the key in the start position. You should see 12V there to energize the solenoid. If not, start working back from there.
If you dont have a volt meter, or dont know someone who has one, start with the basics. Clean terminals and make sure they are tight. Check all safteys and make sure they are making contact and plugged in. Neutral saftey, seat switch, pto switc, etc. If you still can figure it out, without a volt meter, you are probabally going to either need to buy one or take it to the dealer.