Can anyone tell me if there is a trick to cutting a mirror

YES, patience, practice and a good glass cutting tool what type of mirror i.e. how thick how big and are these straight line cuts.

Actually I'm going to buy a new mirror because I won't pay what a mirror shop wants for one. I can't find an exact size si I'm going to buy a cheap one at Lowe's and cut it down. It a relacement for a bathroom medicine chest. So your saying cut it the same as glass? Oh it looks to be maybe 3/16'' thick

A mirror that thin should cut just like glass with a good cutter. How will the edges be finished does the mirror mount in some type of frame. The cut edges will be sharp!
In the interest of safety might be a good idea to wear gloves and goggles . If you have any other questions post back . GOOD LUCK , AC

Only cut a miror if the edges will be covered by trim. Your cut will not be factory smooth and the silver on the backside may be a bit jagged. If you want to get a factory edge you would need to sand and it could be tedious.