According to the voltage drop calculator I used at this sight:
You need a #4 copper wire in conduit to supply a 240 volt, 60 amp line the 125' to your garage. You will need a 60 amp double pole breaker at you main panel. Then you will need three #4 copper wires 125' long and 125' of PVC conduit and fittings. I think you will probably want to use 1 1/2" conduit. You might get by will 1 1/4" but don't try to get it to snug, it is not worth the fight.
For a ground you will need an 8 to 10 foot copper ground rod (check local requirements) with a #6 bare copper run from the sub panel to the ground rod and a ground clamp to attach the wire to the rod. Use 1/2" pvc conduit to protect your ground wire from the time it comes through the wall until it goes below the ground. To install a ground rod in my area the easiest way is to dig a 10 to 12 inch hole where you want to put the ground rod. Then pour 3 or 4 inches of water in the bottom of the hole. Push the ground rod into the ground as far as you can then pull it up and let the water run down the hole you just made. Push the rod in again as far as you can and remove it again. Repeat these steps and the rod will keep disappearing into the ground. After a couple of feet you do not need to remove the rod from the hole just keep working the rod up and down while keeping some water in your original hole. The entire length of an 8' rod should install in a few minutes. Set the top of your rod below the surface of the ground and bury the whole thing after your ground wire is hooked up.
Good luck and let me know if something is unclear.