It depends upon your motor. Vaious motors are designed to run on 1.5 V, 3 V, 6 V, 12 V, etc. Of course, you can run a 1.5 V motor at 3 or 6 and it will go very fast, but it won't live very long! You need to find out what yours is. If you are only outputting 3 V, a 6 V motor will turn more slowly than you want and a 12 V motor will probably not even start.
Do you have a motor? If so, hook it to a power supply for some batteries and find out what voltage you need to run it at the speed you want. If that is over 5 V, you will either need to use a different motor or use a higher voltage power supply. Let's say you want to run a 12 V motor. Your picaxe will run on 3 to 5 volts. It will control a transistor and the transistor will switch the 12 V power supply for the motor.
If you need help doing that, let me know.