How to reroof my garage?

Be sure to take off all the old shingles before you put new roof material on.

we need more detail... are you replacing just the shingles, or the whole roof? how big is this garage? where do you live (what's the weather like)?

I would look at some of the great articles on the "This Old House" website. I saw an episode of "Ask This Old house" where they walked through the whole process. This artical from their website might help. http://www.thisoldhouse.com/toh/knowhow/design/article/0,16417,1205780,00.html

Also check out DIY sites for reroofing, and Lowes and Home Depot have instructions on doing this too.

Have it check by a roofing contractor and have it estimate. If you don’t have much budget, ypu can check some DIY websites or youtube videos that can help you out on this.