If most of your riding is on paved surfaces, you want "city bike tires." For your bike, they'll be roughly 26x1.5"-2.0" and they'll be relatively smooth instead of knobby. Better ones will have features like extra puncture resistance and retroreflective sidewalls.
Random example from an image search:
The "some nubs on the outer sides but a smooth patch down the center" type of tire the commenter above recommended would look something like this...
...but honestly, I'd probably go for a full-blown city tire instead unless you're regularly riding on loose surfaces, not just "occasionally."