The first step is to figure out exactly what motor you have.
There should be some number or something on the motor, hopefully - either printed or written on the housing somewhere, or possibly on the "end bell" (rear) of the motor (you may have to look carefully at this, using proper lighting, as it is generally molded or embossed on the end bell housing). At a minimum, you should be able to find the manufacturer - which then may lead to the specs (go to the manufacturer's website, and do some searching there for spec sheets of similar motors - or, in a pinch, take a picture and send it to them asking about its specs).
Which leads me to wonder why you haven't posted a picture(s) of the motor to this thread - so we could possibly help you figure it out?
Last - to determine the stall current (which will vary depending on the voltage applied) - put a multimeter in series with the motor and battery: First, set you meter to measure current, and put the probes into the proper jacks on the meter for current measurement - then, hook up the meter in series with the motor - battery + to motor terminal, other terminal to meter, other meter terminal back to battery.
The motor should run, and you should see a current measurement (it will fluctuate some) - note that current (this is "no load" current) - now load the shaft down, but don't stall it - and note the current again (it will rise) - this is the approximate "running" current.
Finally - and most importantly - you need to get the "stall" current. This is best done by measuring the coil resistance, then using Ohm's law with your expected voltage - so disconnect and put your meter in resistance measurement mode, and with no voltage connected to the motor, measure the resistance between the motor terminals. Then (if you can) rotate the motor's shaft, and measure again. Do this a few times, then take the average value. Use this value for your calculations.
Do each of these things for 3 volts, 6 volts, 9 volts and 12 volts.
As you run the motor, note the amount and "quality" of noise coming from the motor. Also - by touch (or by an IR temp probe, if you have one) - note the temperature of the motor housing as it runs. If it seems like it is too high, or it is making weird noises, decrease the voltage. Also note the "smell" from the motor - if it has a large ozone smell, then excess sparking of the commutator is happening, so decrease the voltage. All of this can help you determine (at least get an idea of) what the highest voltage the motor should bel run at, and the amount of current it will draw for that (and other) voltages - so you can size your bridge/controller properly for the motor.
Finally - note that if you motor (for instance) needs 12 volts and draw 5 amps (at that voltage) - don't go out and get a "5 amp" controller - you want to size your controller to be 10-15% larger than what your motor will draw. So keep that in mind as well.
Good luck.