Rooting is irrelevant (and unnecessary) for customizing. If you just want to change how the phone looks, try WidgetLocker as a lockscreen replacement and LauncherPro or ADW Launcher as a replacement home launcher. Pick any random clock widget (Beautiful Widgets, Fancy Widget, Make Your Own Clock, Minimalistic Text) or Desktop Visualizer to make custom icons and you can go crazy... all without rooting.
All of this... not rooted.
Just about all of the screenshots in this and other forums show different apps, widgets, and wallpapers. Whether or not they are rooted, they all use basically the same customization features.
One of the only things that rooting allows for visual customization is changing system fonts and colors. But there is so much you can do without being technically inclined. You can't break it. Don't be afraid to change things. But most important... have fun with it!