There are many possible configurations and degrees of complexity to rainwater systems. Costs vary considerably as well. You can spend anywhere from a few dollars to a few thousands of dollars.
Your best bet is to review the options available on the market to find out what’s in your price range and what’s a realistic set-up for your home.
The simplest use of rainwater if you are on a budget or have space restrictions is to put a rain barrel under one of the gutter downspouts and use the water on sensitive indoor plants.
For a slightly more sophisticated system, use several barrels connected together near the bottom with PVC pipes or hose. A small pump can be used in one of the barrels to pump the water to your garden. In this case, all the barrels will drain simultaneously.
Bigger and more complex systems may use gravity to feed water from gutters to a larger cistern, which pumps water to the landscape. Some online gardening sites sell cisterns and other more complex rainwater harvesting equipment.