At a safe source of fresh water! Just kidding.
Find the tank(s) that are for fresh water. The fresh water pump may be nearby so you can locate them when it is running when you turn on the circuit breaker labeled water pump (or similar). The tank is probably under a bunk and a cover may need to be removed to view it. One end or the other should have a 1 1/2 inch diameter line going out and up from the tank. That is probably the fill line headed for a plate on or near the deck. Many fill necks are on the side deck area and may have lines leading up through the head area or a hanging locker/closet. Our Caliber has the fill in the bow anchor locker since the water tank is under the v berth. That is not the most convenient place for a fill since I need to sit and lean forward while it fills.
The water fill could be in a cockpit coaming locker. Just a thought.
MOST IMPORTANT: Label each fill neck so you do not confuse fuel fill, water fill, and waste pump out. Doing the wrong thing at any of those openings could ruin your day!