hi and welcome to the dark side...
I am assuming you have a Sprint HERO on CDMA network.
here is a link to a lot of reading you will need to do:
[REF] ALL ROMS | ROOTING | FAQ's | STICKIES | POSTING RULES - Read before posting - xda-developers
you will need to learn about
rooting the phone
to do a Nandroid Backup in Recovery Mode
then flashing the new custom ROM
advice: do not flash the new Leak Sprint 2.1 ROM.
i think there are a lot if issues that a NOOB will have hard time handling. Let one of the great developers, make a custom ROM of it first and tested for your safety.
Use one of the tested Android 2.1 Custom ROMs. example from DamageControl or Flipz.
I am using DC 2.08.1 and it is stable, fast, and everything is working.