They can get through REALLY small holes (like the size of a penny) so you have to walk around the outside of your house, looking for potential openings. Note things like -- lack of screen over the vent for the dryer, cracks beneath porch or steps, slight opening below the garage door where it doesn't hit the driveway below, etc. I have a mail slot opening that the flip covering fell off of. I didn't fix it immediately and found that's how the mice got into my place.
Once in, they breed really FAST. Unlike most small mammals, they are in estrous 100% of the time (rather than at only certain times of the year). The faster you act the better you'll be able to handle the problem.
I found a no kill mouse trap really works great to catch them. They're under $15 and you can re-use them as much as you want (unlike conventional kill snap-traps). As long as you release the mice a hundred yards away, they won't be back. See these at Amazon here.