Search the Arduino site..
(or google)
In the IDE, there is even example servo code to use/test with.
There are also some posts around here too.. (but dealing with the bucket/helmet and not an arm launcher).. but same concept applies.
Some tips:
1.) While you can DIRECTLY connect the servo to the Arduino.. (for testing).. this is NOT recommended under any type of LOAD (ie: moving anything)
A servo will draw upwards of 1 amp in current.. and an Arduino pin can really only deliver 20-40mA (milliamps) safely. (otherwise you'll ruin your Arduino)
So you will need to power the servo from the same battery pack that is powering your Arduino. (you might be able to power it from the VIN pin, but look into it first as I dont recall if this is safe off hand or not)
2.) Depending on the servo.. you will need to regulate the voltage (usually between +4.8v - +6.0v) so you'll need a separate voltage regulator..
Basically the only thing connecting the servo and the Arduino will be 1 wire (the control/signal line/wire)..
3.) Dont forget to connect ALL GNDs together.. battery/servo/Arduino (this will help if you see 'jitter' in the servo while powering it)
This is a pretty basic Arduino lesson, so there will be TONS of it!
Good luck.
