Turn the key to the ON position with the car in neutral.
Take a big screwdriver and ground out the positive terminal on the starter to the the other two posts.
If it sparks a bunch, but does nothing, something in your starter has crapped out.
If it turns over the motor, something up stream of the starter motor is wrong from the key to the starter. Possibly a main fusible link.
If it does nothing at all (no sparks), there’s a problem getting power to the starter. The voltage on the positive terminal of the starter (where the big + wire goes to off the batter) should be very close to the same voltage you get at the battery.