Dislocated finger (pinky) during volleyball. how can i keep it from getting worse?

Protect then move it: Xray is a good idea to confirm that the dislocation is reduced fully and that there is not a fracture. If that looks ok, 7-10 days of immobilization in a splint with occasional removal for cleaning and gentle range of motion. Then more aggressive stretching and strengthening exercises with the finger taped to the one next to it above and below the affected joint ("buddy taping").

Which joint?: It is important to know how it was dislocated (which direction) and then tailor treatment appopriately.