As I mentioned in a comment to Jay's post, rainwater collecting may be illegal, dependent on your State. Western states, which get less rain, generally employ the doctrine of "prior appropriation"; an entity can claim the rights to water that will enter a waterway even if it hasn't arrived there yet. This means that by collecting rainwater into a cistern, and thus preventing it entering a stream, lake or aquifer, you have prevented the claimant from receiving the water they were promised by their permit and are breaking the law.
The dividing line between states that do and don't employ this doctrine is generally the Rocky Mountains, though a few Plains states prohibit artificial collection of rainwater without a permit. Most Eastern states have more "riparian" doctrines (you are free to access any water that falls on, collects on, borders or passes by your property), with more or less restriction based on type and amount of use and current drought conditions.
In Ohio, rainwater catchment is legal; however, catchment and cistern design and construction is among the most highly-regulated of all States, and while you don't need a water use permit, the catchment system must be permitted by the health department. Have a look-see: http://www.harvesth2o.com/statues_regulations.shtml#oh
Once you've sorted out the legality, Jay's post has the rest of the basics.
While researching, I looked into the laws of my own state and found some pretty interesting facts, which may be entertaining to other readers:
Texas has mostly "prior appropriation" doctrines for surface and groundwater; all major waterways within the borders of the State are owned by the State and administered "for the public good", and it's illegal to divert these waters without a permit. Despite that, an older, more riparian doctrine has persisted with regard to rainwater (called "diffused surface water" in the statutes); landowners may collect precipitation that falls on their property and hold it for domestic and livestock use without any permit, provided you don't have storage capacity in excess of 200 acre-feet.
- 200 acre-feet is the volume of a tank one acre in footprint (660'x66'), and 200 feet deep.
- Putting that in real-world terms, one acre-foot is 325,851.429 gallons, or the equivalent of about 3 and a half years' worth of water for a family of four (everything from cooking and drinking to washing dishes, doing laundry, taking showers and watering the lawn).
- The big hilltop tanks you see that provide water pressure for mains and hydrants (and bragging rights for local high schools' athletic teams) top out at about 12af (that's a tank about 1/3-acre in footprint and 35' tall not counting its pedestal).
- Going by average rainfall in the Dallas area of 38"/yr, it would take approximately 63 years for 200 feet of rain to fall on 1 acre of land.
- If you could catch every drop that fell on one acre in an average year (and prevent it evaporating again), you'd have just over 3 acre-feet, enough for about 10 years' average use by a family of four.
- That means that being able to catch every drop on just about any plot of land worth owning (1/10 acre is practically zero-lot-line housing, like townhomes/row houses with front and back yards) would be enough to live on.
- Your average suburban homeowner has somewhere between one-tenth and half an acre, requiring a 200af tank that took up every available square inch of their property to be somewhere between 400 and 2000 feet tall (or deep).
- More realistically, a tank holding about 1/2af (a quarter of one percent of the limit) would have a volume about 20,000 sq ft; that would be a tank 55'x40'x10' (a pretty big swimmin' hole, but possible on a larger plot) and not counting evaporation loss would hold more than enough water to keep the average family of four going for a year.
The limit is thus something the average homeowner NEVER has to worry about. Catch as much as you can in Texas.