moving an eighteen foot tree is a complex job. You would need professionals and probablly a crane. They would come out about three feet from the stem and dig down about 4-5 feet. The stem would have to be carefully protected so the lifting wire or rope will not strip the outside layer of the stem. The root ball is either a square or a circle which at 6' across and 4-5' deep is alot of weight. Not an easy job.

Mine is only 7 foot tall and was dug out when I just bought them. How do I prepare the ground and do I need to do something special to avoid shock

a lot depends where you are and your typical fall weather

A pic would be nice. I'd probably leave it, build the garage. Then depending on how much damage it sustained from the construction, you could trim it way back next spring to reshape.