The first step is usually to check for power. Measure your battery voltage. It should be about 12.6. If voltage is good have somebody monitor it while you engage the starter. If it stays the same you're not completing the circuit. Do you hear a click or any noise when trying to start? You may not be satisfying one of the safety circuits. More information about your issue could be helpful.
Had something similar a few years ago. There was a wire that corroded apart under the insulation. as I was starting to look it over to trouble shoot, I felt the "flippy" section of wire. Trying to remember but I think it was a short wire that went from the red wire to the solenoid and I eliminated it and put a StaKon on the red wire to connect it.
When I turn the key there is nothing, no clicking. Battery is brand new and is fully charged. Two lights on dash come on; one is battery light I believe the other I’m not positive. These come on when key is turned before trying to engage the starter.
My 1210 was parked after a snowfall and when I went to use it - maybe a couple weeks later - I thought the battery was kaput. That wire on mine may have been a one in a million event. It would be an easy thing to go tug on the wires to make sure you're not having the same problem.
If you still need to go searching, one of the most useful testing items is my Harbor Freight 12V test probe.