From the Apple Help System -
*Aligning, labeling, and resizing icons*
To modify icons:
To modify icons in a Finder window, open the window.
To modify icons on the desktop, click the background area of your screen.
Choose View > Show View Options and make your choices.
Each of the views—icon, list, column, or Cover Flow—has its own view options. The icons change as you make choices, so you can immediately see the effect of your changes.
“Snap to Grid” (available in the “Arrange by” pop-up menu in icon view) makes icons align in rows and columns when you move them around.
To apply your changes to every window you open, click “Use as Defaults.”
This option does not appear if you’re adjusting icons for the desktop.