Quite possibly the belt. Also the easiest to check and fix, so eliminate that first.
Quick test: before starting pop the bonnet, check how to get to the belt and have a can of WD40, with extension nozzle fitted, in hand. Start the engine, listen to squeak, squirt a bit of WD40 onto the inside of the belt or the contact face of a pulley (obviously keep ties, fingers, etc, out of the moving parts). If the squeak stops - it's the belt. Then decide if it needs adjusting or replacing (condition? tension?). If the noise doesn't change it's something else, maybe water pump or alternator.