What Resistor Value to Use for Running 12V Motor on 18V Battery in Battlebot IQ?

i am working on a battlebot project in my high school shop class and was wondering what type of resistors we will need to run a 12v motor off of a 18v battery. without ruining the motor
i know nothing about electronics so any help would be appreciated

Running a single resistor in your design is not advisable. Running a voltage divider circuit would be ok, but keep in mind: when your battery is weakening, the resistors are still dropping voltage in an already weakened battery.

I suggest that you make use of the forward voltage drop in a silicon diode such as a 1N4001. It's around 0.6V, so to give the voltage you need, use 10 diodes in series which will give a voltage drop of 6 volts, leaving you with a voltage of 12 volts which will not be affected by the current flowing.

Based on your 18V battery i think you want to use a power tool battery.

Unless you are using a PWM controller to step down the voltage you will just burn energy.

( to know the resistor Google for OHM s law you need to know the running current from the motor and you voltage drop(18-12=6V), so lets say running current is 4 Amp then 6/4=1.5 Ohm but you also need to know its power rating so you take the voltage drop times the running current (6*4=24Watt) in this case you need a 1.5 Ohm and minimum 24Watt resistor, so you burn your energy as heat)

The best solution is to use a sealed gel (AGM) 12 Volt battery.
P.s. if you use a remote control there is a good change that it has a speed control (PWM) already build in and that it works from 12V.