I don't often recomend Home Depot but, I did brake down and buy "Wiring 1-2-3" for $15 to help me with my three way wiring.
Wiring 1-2-3 states;
First find out where the power is coming from.
It sounds like the power is coming from the garage switch. If this is the case, in the garage, you should have a white, black, and a ground coming in and a white, black, red, and a ground going out.
In the house, you should have the white, black, red, and ground coming in and a white, black, and ground going to the lights.
Turn the power off at the circut first!
At the first box (garage), splice the two white wires together and connect the black,(from the two and the ground) to the common terminal, and connect the grounds.
Then you should take the black and the red wires to the traveler terminals.
At the second swith box, connect the black wire coming in from the fixture to the common on the 3-way dimmer, splice the white wires together, and the remaining red and black wires to the traveler terminals, then connect the grounds.
Turn on the power and test.
You need to make sure the 3-way dimmer is rated for the total wattage of your lights.(example,a 600 watt dimmer switch can't handle 8-100 watt lights).
You should buy the book it is very well illistrated and I have found it helpfull.