I think kitchen sinks are 3/8" (but I can never remember). So you'll want a 3/8" female, 1/4" male, 3/8" male tee. You should be able to find one at the local big box, or hardware store.
Something like this
- Shut off the water using the shutoff valve.
- Remove the existing supply line.
- Attach the tee where the supply line used to be.
- Connect the sink supply to the 3/8" male outlet on the tee.
- Connect the refrigerator supply to the 1/4" outlet on the tee.
NOTE: Use your preferred pipe dope method (tape or goop), however, since it's a brass fitting dope should not be required.