There's a tutorial by Duvik that sets up a grid in Photoshop and another that builds a dungeon on that grid. I suggest that you start there.
When setting up your document, decide on your print resolution. 150 pixels/inch is probably a decent resolution for your purposes. When you make the grid, make sure the size of your squares equals that resolution. That way, you'll have, for instance, 150px squares, so when you print at 150dpi, you'll get 1-inch squares.
And a heads-up: When you present your finished maps, be prepared for people to ask for a version without the grid. Virtual Table Top (VTT) software generally makes its own grid, so many posters here like to see "clean" maps that they can use in MapTool or Viewingdale or what-have-you.
Good luck building your tiles, and have fun!