Find Hopper8's 'Odin troubleshooting' guide stickied in the General section. Download the 3 part stock firmware linked in the thread, flash with Odin (this will wipe your phone). If you have problems with the flash, do the stuff suggested in the thread until you get a successful one. This will give you a 'clean slate'.
From there, you can re-root your thread & flash a custom rom/whatever. How ? Search. 'How to root' has been asked/answered thousands of times here over the past two years; one hint I'm giving you: Thread stickied near top of Q&A has simple root method (others in guides/tutorials stickied near top of General section). So far as flashing a given rom is concerned, flashing instructions are always on the first page of the thread devoted to the rom, and are clear/concise 99.9% of the time.
Best bit of advice I can give you - don't do anything to your phone until you understand what you're doing. Not doing so leads to busted phones. Take your time.
If it's important enough to cause you grief if it's lost, back it up. No excuses
Trolling for thanks is lame
Does Samsung's auto-brightness implementation crap drive you to self-harm ? A solution !
Failure to search is the leading cause of male impotence on XDA
If you don't know how to recover if things go wrong, what are you doing flashing custom roms ?