This site will give you everything you need and TONS more:
http://forum.xda-developers.com/forumdisplay.php?f=1790
This guide tells you how to root and reset the flash counter on stock rom:
http://forum.xda-developers.com/showthr ... ?t=1896696
There is lots more reading, you'll find with many of these things, it's not always simple/easy unless you a have a BIT of command line/driver experience on your computer too.
The important bits:
EFS backup - you'll see this, a lot. DO IT. EFS contains your phones IEMI number. If you wipe it (possible when rooted) then you are TOAST if you come to sell your phone in the future. If you send it to one of those 'trade your phone in' places, they will KEEP it and you will get NOTHING. Zip. NADA. SQUAT. You can't even get it back.
BACKUP EFS DATA
The tool you are using is ODIN, it writes the whole phones OS/SYSTEM/ETC - get the wrong flash and the phone might not start again.
Understand the flash counter and learn how to reset it. IF you don't and need to send your phone back, they will check this. If the phones 'flash counter' has incremented, then they will more than likely wriggle out of the warranty claim = no phone or pay them mega pounds to sort it out.
Having root can mean some apps don't work - check them out first, things like banking apps may check for this. If root is detected, then they may be blocked.
Under pretty much ZERO circumstances should you download APKs from the web offering 'free' software that you would otherwise pay for - it's almost 100% going to compromise your phones security (trojans, diallers, sending messages, premium rate calls, stolen data).
Think that's it......but there are some cool things you can do once unlocked
Edit:
What are you wanting to do, custom ROMS or other?
If it's root, then this bit:
http://forum.xda-developers.com/showpos ... ostcount=2 and read Method 1:Root with stock Recovery
If you want to flash ROMS and stuff, then you are better off with CWM and root (clockwork mod - a way you flash custom ROMS and backup your device).
BG