The basic idea in most AG brewing is to use specific quantities of water in your mash (when you soak your grains in water at a specific temp to convert the starches in the grains to sugars) and your sparge (when you rinse the grains to remove as much of the sugars as possible into the kettle) to achieve a volume that will boil down into 5 gallons, or whatever final volume you're shooting for.
The exact volumes you will use depend on a lot of variables, including but not limited to:
-the water-grain ratio you want in your mash
-the temp you'll be mashing at
-how many steps you'll use and whether you raise temp by infusion or not
-what kind of sparging technique you use (if any)
-how long your boil is
-what your boil-off rate is
-how much loss you expect (due to mashtun design, grain absorption, etc)
Brewing software is probably the easiest way to streamline these variables as most of the relationships are modeled and all you need to do is enter your parameters. I use both Brewsmith and Hopville.com (which is free and easy for quick recipe mockups and calculations), others have their favorites. But either way, it's good to understand what the computer is calculating, even if you're not running the formulas yourself. If you don't have a basic brew book like Palmer's How to Brew or Papazian's Joy of Homebrewing, they give good overviews of the AG process that should get you started. If you have other questions, there's a wealth of knowledge on this site.
Cheers