See a doctor. Any shoulder pain—especially that from an acute injury—needs to be evaluated by a sports doctor.
Employ dynamic rest. Lay off the upper-body work (including swimming), and use lower-body workouts to maintain fitness.
Ice it. Ice applied to the shoulder for 15 minutes several times a day can help reduce inflammation.
Try an NSAID. An anti-inflammatory like ibuprofen or naproxen can help with the pain.
Start rehabbing. As the pain improves, do some rotator cuff exercises to help strengthen your shoulder. See below for several suggestions.