Honda Oddyssey won't start

All electrical functions will work even with a weak battery. It takes a lot of amperage to start the car.
Have you tried jump starting it from another vehicle?
If the starter is still not turning, then check for loose or corroded battery cables.
Still no movement? Have someone hold the key in start position and tap the starter with a small hammer. If it starts, you have a bad starter. But confirm the battery is ok first (most common).

Starter is located at top of transmission, towards left of transmission housing if you are looking from transmisison end.

If you follow the battery positive cable, you should come to it at its end.