Mice can squeeze into your home through the smallest of holes. An easy way to keep them out is to fill all your nooks and crannies with steel wool. You might want to take the extra precaution of also filling small openings in the outside walls with steel wool.
While going through this process it’s inevitable that you’ll miss a hole here and there so I would suggest storing all food in your cupboards in recycled, reclosable plastic containers and/or coffee tins. And of course set out a few baited traps.