Determine what the rainwater will be used for. For watering the lawn and garden or washing vehicles, rain barrels placed under the downspouts of your house are what you need. The water can be stored in the barrels until used. If you intend to use it in the house for washing cloths, flushing toilets and bathing, you need a more elaborate system. The rainwater should be channeled to one downspout that drains into a larger tank which is connected to the plumbing of the house. The rainwater is stored in the tank until used.
Determine the location of the tank in regards to the house and your needs. The tank can be above ground or below ground, depending on your preference and budget. Dirt and debris should be filtered away before the water gets to the tank. You can periodically test the water for bacteria or other pollutants that would make it unsafe for drinking.
Make sure the materials in the harvesting system do not contaminate the water with harmful substances. For example, an older roof might have been built with asbestos materials and the flashing could contain lead. As the water runs across these areas, it could pick up these harmful materials and be contaminated when it reaches the barrel or tank.
Ensure the materials and coating used to build the storage tank have been tested to ensure they do not release anything harmful into water being stored if you intend to use harvested rainwater for drinking. The manufacturer should provide this type of information.