There's several ways to go about making your phone more to your preferences.
Probably the easiest thing you can do is to swap out the launcher, to something like ADW Launcher (either the free version or the EW version) or LauncherPro (again, either the free or the paid). There's several more options there, but those tend to be the most popular two.
In order to make major changes though (like making the battery/notification bar/etc look like the Nexus S), you'd need to root the phone, and that's not something you just want to jump into without researching it first. Don't get me wrong, that'll open up all sorts of options, but it can't be understated that you're voiding your warranty to do so.