Many things to consider.
Has the garage got mains electricity?
How many points of entry - doors, windows?
Draughty? Damp? Hot? All these can cause false alarms if detectors not carefully chosen.
The audible alarm ("bell box") will inevitably be low down, so it is easy for an intruder to attack it. Again, the spec of the bell box needs careful consideration.
What kind of risk level? Is it just a few diy tools and family car to secure, or something far higher value - which could attract more expert thieves and need higher level of equipment.
If the garage is close to or integrated with your house, it might be an option to put control equipment & sirens in house, not in garage. Radio links are available if not possible to hard wire.
Is there a requirement of alarm by insurance company? If so you may need professional NACOSS recognised installer.
Is CCTV an option?