Rainwater collection can be simple or complicated depending on whether or not you've planned for it before building or are retrofitting an existing structure.  I recommend searching Amazon for books on rainwater collection, such as Design for Water.  While this book's information on rainwater collection is very good, it's information on rainwater purification is inadequate.

Rainwater purification has grown more complex due to the spread of pollutants on worldwide trade winds.  Mercury from coal plants in China spreads to the entire Western US.  When collecting rainwater or spring water plan to purify for bacteria and the presence of mercury and pesticides.  Rainwater purification and rainwater filtration need not be complex or costly, but treating rainwater must be thorough enough to account for the pollutants likely to be present.  Mercury and pesticides are now present even in pristine environments requiring rainwater purification even in these locations.  Purifying rainwater is similar to what would be done for mountain spring water.

If you're planning to drink the water then my 4 stage Kitchen Defender with UV light is perfect for purifying rainwater or spring water.  In remote areas with no electricity you can use my 3 stage Kitchen Defender but you'll have to power a pump or use gravity to get the water to flow through the system.

Kitchen Defender

with 5 micron filter, KDF/Carbon,

UV light, and Carbon block

Request specifications and pricing on the Kitchen Defender

To preserve the integrity of the Kitchen Defender from sediment you'll need to use a whole house 5 micron filter ($136).

I also have a commercial version available which will handle higher flow rates but still only for kitchen use.

For whole house rainwater purification you'll need my Urban Defender and UV light.

Collecting rainwater will be unique to each building site but I've included a sketch and a couple of photos to provide some ideas on the approach.  You'll need to collect rainwater from a rooftop, filter the large debris and the initial wash, then store the water, and then pump it to your rainwater purification system.  Filtering rainwater with a large micron filter is a good idea but treating rainwater should take place after storage because it will be virtually impossible to keep the stored water contaminant free.

Also available as a pdf: rainwater collection system

Here are some ideas for collecting and purifying rainwater:   rainwater collection sketch

DENVER (3-25-09) — Although rainwater collection is drawing new interest as an onsite water source, harvesting rain is against the law in two states and is limited in a third.

Colorado and Utah prohibit rainwater collection, and Washington State allows it only in a few regions such as Seattle and the San Juan Islands, the Journal article said.

In the belief that such a ban makes no sense and is virtually unenforceable (penalties are vague in the law and rarely meted out), two Colorado legislators, state Rep. Marsha Looper (R-El Paso) and state Sen. Chris Romer (D-Denver), are trying to lift the ban on rainwater collection in their state.

The Wall Street Journal notes that rainwater collection bans originate in the West’s complex water rights laws. In Colorado, the law says that every drop of moisture in the atmosphere over the state is accounted for — those drops must be allowed to hit the ground and flow to various watersheds, where the water is divided up by law among the state’s many water users.

Critics note that people who filter rainwater for drinking will probably direct most of it back into the ground anyway close to where it would have fallen, to water plants and crops. But “powerful forces” in Colorado still don’t want any of it diverted, the article said.

Looper said she would like to put out a few rain barrels to grow hothouse tomatoes, but she won’t do it because “I don’t want to get thrown in jail.”

A recent attempt to lift a ban in Utah died in that state’s Legislature.

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