fix broken water pipe in garage (burst in winter)
You'll need to open the wall to figure out where it's broken and try to see if it's in multiple spots. Depending on the piping you have and exactly where it's busted, you might be able to fix it yourself with some SharkBite fittings and new pipe. Either way, you'll need to start by opening the wall and locating the break(s).

As for preventing this issue in the future, you'll need a way to drain the pipe. The normal way is to put a shutoff value and drain valve in the warm area (your house) on the pipe that goes to your garage and use that to blow out the pipes with air. In the warm area, you'll want the drain value on the side of the shutoff value that is closer to your garage. Then once your garage line is blown out, you can leave the drain valve open until you're ready to recharge the line. Be sure to blow out each fixture in the garage and put anti-feeze in the drains, toilet trap and some in the toilet tank as well.