It also depends upon how much current your motor draws and how much your transistor can safely provide. A typical small transistor can maybe run a little toy motor, but it isn't going be able to run a car starter motor.
Hook your motor up in series with an ammeter and then to your battery or power supply. Watch for the peak current when you turn it on (do it a number of times) and also the current while it's running. Then choose a transistor that is able to provide that. Actually, you should pick a transistor that can provide maybe 50% more, to be safe.
Also, your meter might not be able to react fast enough to show the actual peak current.
And, if you plan to run the motor for very long, the transistor may get hot. If so, you need a heat sink or a transistor designed to handle a larger current. Or you can get a Darlington which is two transistors; one driven by the other.
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.