Is it just Ohms law?
No, you just supply the gate voltage and it turns on. You need to look at the datasheets
for the MOSFETs, at the curves of Vgs vs Idrain.
Basically, it's easiest to use a logic-level nMOSFET, as it will turn on hard with Vgs=5V.
Then, for the current levels you are talking about, 10A, find one with the lowest Rds
rating you can find, typically something like 0.01 ohm or less. The current rating [max]
for such devices will normally be many Amps, 50-80 range.
Put the diode right at the motor. This reduces the inductive current-loop [motor
winding -> wires -> diode] to the minimum. Otherwise, it's like a big loop antenna,
broadcasting to the outside world.