Inspect the basement to determine its suitability for a gym. Check the basement for flooding after a heavy rain. Inspect the walls and floor for signs of damage such as cracks. Measure the height from floor to ceiling to ensure you have enough room. If the ceiling is too low, you might not be able to do certain exercises or install certain pieces of equipment.
Determine your budget for the gym. If it's limited, you might want to purchase equipment that can be used for more than one purpose. For example, instead of getting an exercise bike, which you can only ride on, a treadmill might be a better purchase because you can run, jog or walk on it.
Measure the area of the basement -- excluding objects like your water heater -- to determine how much space you have available.
Decide what type of exercise program you want to do and what equipment you need to get. Again, because of budgetary and space requirements, consider buying multipurpose equipment.
Sketch out the layout of the gym with the equipment you plan to purchase. When planning the layout, make sure that you leave enough space between each piece of equipment that you exercise without hitting other pieces of equipment, the walls or the ceiling.