Since how long are your symptoms?
Is there any swelling, redness, tenderness?
Any history of injury?
Are you diabetic or have thyroid disorder?
Are you on any medications?
One of the possibility is trigger finger. The treatment is local steroid shot. Investigation for the thyroid and diabetes should be undertaken. Anti-inflammatory analgesic like Ibuprofen (Motrin / Advil) are also helpful. Please read about the trigger finger;
The other possibility is tenosynovitis. It is inflammation of the tendon and its sheath (synovium). The treatment is warm moist compresses or moist heat (warm water insertion of the finger, thrice a day), local analgesic cream and Ibuprofen (Motrin), with mild physical therapy.
Please feel free for your follow up questions.