What version of the OS are you running?
Assuming it's either Lion or Mountain Lion you would boot into recovery mode (hold the command and r keys down while you boot the machine) and follow the screen prompts to install the OS. You're just looking to redo the OS no disk format or anything of that sort.
While installing the OS over your current system is normally non-destructive to your user data it is of course highly recommended that you have a current working backup just in case.
And while you don;t need to be told not to do this again remember the system files and programs should be left alone. First you never know when you might what something, second you never really can be sure how things tie together so removing something can have unforeseen consequences and third you really don;t save much disk space removing these things.
Once you sort out this issue, see these:
Switching from Windows to Mac OS X,
Mac Basics—Tutorials on using a Mac,
Mac OS X keyboard shortcuts,
Anatomy of a Mac,
Switching to Mac Superguide, and
Switching to the Mac: The Missing Manual, Mountain Lion Edition.
Additionally, *Texas Mac Man* recommends: