Could be alternator. Could be regulator. Belt as suggested. Battery itself. Wiring/grounds.

Do some tests and narrow it down. Test your voltage at idle. Test your voltage at higher engine RPM. Do these tests before the regulator, after the regulator, at the headlights. Do them with headlights on, and headlights off.

All of this info will give you better clues of where exactly to look for the actual problem and solution.