Icons are a popular thing to change when you are looking to customize Mac 10.6. There's not much to it because changing an icon is really just a case of copying and pasting. First, find the icon you want to use, click on it, and then press Command+I on your keyboard to open the Get Info screen for that file. On the info window, click on the icon in the top left hand corner, and press Command+C to copy it.
Now find the icon you want to change. Click on it and press Command+I again to open the Get Info window for that icon. Click on the icon you want to change in the top right hand corner of the window, and then press Command+V to paste the icon that you copied moments before.
You can copy and paste icons that are already on your Mac, or you can browse the internet for icons to download for your Mac. Try the Icon Factory or Iconspedia for a selection of good quality icons. Alternatively, you can make your own. Almost any image you have can be used as an icon. Just open it in Preview and click Command+C to copy it. Then click the icon you want to change, press Command+I on your keyboard and then click on the icon in the Get Info window. As before, paste the new icon by hitting Command+V.
You can change the size of icons on your Desktop and in your folders. To do this, right-click (Control+click) on your Desktop then select Show View Options. Move the slider to change the icons size how you like it. If you have a very few icons on your Desktop, try maximizing the size. You'll be surprised at how good this looks.
For a more automated approach to customizing your icons, you could invest $29 in software like Candybar, or try the free LiteIcon.