You can only use the motors that come in the kit of parts, so you have to figure out a way to make one of them work. Also there are rules about what you can and cannot modify on the motors and built in gearboxes.
REad the FIRST Guidelines, Tips, etc manual on
this page it will give you information about the motors, how much torque they can produce, how fast they turn, etc. Ask a mentor about how to understand the numbers, or maybe search on this forum for where someone else has already asked the same questions you have.
You probably will need some type of additional gear or chain or other kind of speed reduction to get the motors you have available to be able to lift the arm without breaking or overheating.
Also look for pictures of robots from past years competition, such as 2005, there were a lot of arm type robots that year.