How do I wire a new out side light on my garage
Welcome to the site:
Do you have power in the garage now?
Is the inside of the garage finished?
If you got access to the attic you might just take power from the overhead light, depending on how that light is wired.
I think you can use a pancake box, just cut out siding and screw it to the sheeting. You may be doing some drywall repair.
There are some variables involved in your project.
1. Will you be incorparating a separate switch into your light circuit?
2. Will your light be regulated by a photocell or a motion sensor?
3. Do you have access to a power source in your garage attic or some place close to light or switch location? (Only needed if you will use a separate switch for this light.)
There are other questions based on the answers to these. This is not complicated but lots of possibilities.