This is likely to be a bit of an approximation, but estimate about 20-25 gallons per person per day. You can go with more like 15 is you use low flow shower heads, a high efficiency dishwasher and washing machine, and are very good at turning off hot water when not needed. If none of these apply to you or people take more than 1 shower a day a piece regularly, go with closer to 30 gallons per person per day.

Look to see if you have a good southern exposure to the sun. You don't need to live in Arizona to get enough sun to make this work, but having a southern-facing surface that doesn't get a lot of shade is important.

The solar section of this website will soon have a tool to help you get a rough idea of what you might need. As a general rule, it is suggested that you have about 10 square feet of collector surface per person in the house. In general you want about 1.5 gallons of storage for each square foot of collector area. You also want to look at the difference between flat place and evacuated tube systems. The former is better in warmer climates as a general rule and the second is better in cooler climates and can achieve higher temperatures overall.

Talk to local plumbers and solar specialists to get specifics regarding your system. They will be able to do precise calculations for your installation and assist you with local building codes.

Get the system installed and enjoy cutting out a chunk of your fossil fuel usage (and costs). Be sure to look for local and federal rebates for installing a solar system.