If you are looking at rainwater harvesting it is important to decide what the water will be used for. For example, if you just want to use the water in the garden then you can just use some pre-tank filtration and a storage tank. However, if you are looking at using the water in the house it is very important to control the level of sediment, address the corrosiveness of the water and biological activity in the water, again depending on the use this will differ. If the water is not for cooking and drinking just for emergency use removing dust and other particles from the water with sediment filtration after the tank and a UV in the tank should be sufficient. If the water is to be used for cooking and drinking then the level of biological activity in the tank needs to be kept low and depending on the setup selected for treating the tank water an additional point of use filter might be needed.
Have a look at the “Typical Rain Water Setups” for some guidance on how to set up a proper rain collection system.