Find the floor joists in the room before you put the frame of the waterbed together. Typically an electronic studfinder will help you find the floor joists. Turn on the unit and point it over the floor. It will indicate where the joists are. Mark these spots with masking tape on the floor. The main edges of the frame of the bed will need to go over these joists. This will provide the support necessary once the bed is filled with water.
Position the bed in the room where installation will be easier. The bed frame for a waterbed is typically very heavy. Position each piece on the floor first and put it together piece by piece. This makes it easier to slide the frame into place over the joists and helps turn a potentially back-breaking job into an easier task.
One thing that is typically not considered when positioning a bed is the proximity to a faucet. While you can simply run a hose to a sink, placing the bed where it will be easy to access the fill spout is essential. In addition, when possible, a waterbed should be placed above the grade level of the house. This helps make draining the bed easier -- water will flow much better from the bed if it's going downhill.
Waterbeds are large pieces of furniture and can easily dwarf a room. You can save space by getting a model that has built-in drawers. That way you won't need a dresser and there will be more usable space in the room. Position the waterbed in the center of the room. This will give you plenty of space to walk around the bed as well as making it easier to fill or drain the bed when it is necessary.