I assume it is a detached garage, not attached to the house. You would need to run a cable underground from the breaker to your garage. It is a fairly simple process (not a complex list of tasks), but the risks of doing it wrong or of slipping up and electrocuting yourself or creating a fire hazard are rather serious. I also take it from your phrasing that you do not have much experience running electrical circuits. If so, I would honestly consult a pro. Adding circuits, expanding the service from the breaker box, and, indeed, almost any work in the circuit breaker box beyond swithcing breakers on and off are some of the tasks that are routinely left for pros. It would not cost that much from the layout you described (not much exploring inside walls and such), and would be more than worth the insurance that the job is done safely and right.
As for the expense, the expense would depend on two factors: 1) the available slots in your current breaker and 2) whether your breaker is wired to handle the added amperage that the new circuit would draw. If you have additional slots in your breaker, and if your service is adequate to support he new circuit, then it should be a fairly affordable project. It would get more expensive if they need to replace or rewire the circuit breaker, or if you need to upgrade your service.
[This message has been edited by Lawrence (edited January 17, 2002).]