Alright guys. Now that you have a rooted phone, we can get to the real deal : unleashing its potential.
A ROM is actually what the name says : Read Only Memory. It's the software on the phone. It's basically a .zip file you must flash -- just like a bootable .iso Linux or a Windows CD/DVD installer.
If you have rooted your phone, you may proceed as following :
Go to the Market and search for ROM Manager. You'll have to buy the full version in order to use all of its features, but that's not essential for this step. Hint : do this via a wi-fi network.
Once you have downloaded and installed ROM Manager, open it and allow Superuser permission (that's why you need to be rooted, haha) when prompted. The first on the list will be "Flash ClockworkMod Recovery". Press on that, and the program will prompt you to choose model (actually just confirm it, since just yours will show up). Press that and wait for the process to be done (you'll see a progress bar right in the title of the aplication).
Now, turn off your phone. Go into the bootloader by holding down the specific combo keys required for your phone model. Wait a few seconds, then select "RECOVERY" from the menu (browse the list with the volume keys, confirm with power key). Once Recovery will be booted, you should see a blue menu on a black background (the text used to be green / yellow in previous builds, by the way). Now, if your phone does have a trackball / optical trackpad, you can browse the list using that; otherwise, use the volume keys and power button combination, just as for the bootloader.
FIRST OF ALL, I suggest you do a NANDROID. That's a back-up : complete, full back-up of what's in your phone right now. It's kind of the same with a ghost image for Windows, if you're familiar with that procedure. You have to do this just in case the ROM you're about to flash doesn't work, so you have to restore the back-up (but that usually never happens, since if you got to the Recovery, you're motivated to flash a different ROM). Make a NANDROID by going to "Backup and restore" and select "Backup". The entire procedure, in the new ClockworkMod 18.104.22.168 should not take longer than 7 minutes -- but that may vary, depending on how much media etc. you have in your phone. Once the back-up has been made, you might want to try it first, just to make sure it's actually WORKING. Go to the same "Backup and restore" and select "Advanced restore" -> "Restore boot". This will restore the smallest portion in the back-up, which should not take more than 3 seconds. You might wanna do this just to check that the MD5 sums are okay, so you won't have the surprise of getting "MD5 checksum fail" when you try to restore an untested back-up at a later time.
Now that you have a working back-up, you're good. Proceed to flashing the new ROM by going to "Install .zip from sdcard" and then browsing to the desired .zip file. Note that some (if not all) ROMs require that you wipe all data before flashing the desired ROM for the first time. The wipes that you will have to do are :
-wipe data/factory reset - this deletes all user-installed applications and reverts the settings back to the original state
-wipe cache partition - this erases the partition which is used for Market downloads (changed location in some ROMs, rendering this partition useless in some cases)
-wipe Dalvik cache (under "Advanced" menu) - this erases the portion of memory that acts like the Prefetch files from Windows. It's similar, and it is recreated at boot (given you've wiped it) -- which could take up to 10 minutes.
So, if you are now on the stock ROM, and you're performing a flash for the first time, you'll have to wipe all those three. I suggest you do it twice, just to make sure everything has been cleared (I do that myself, just a precaution -- call me crazy). Once you've wiped that, you can flash the ROM. After that, simply reboot the system and you're done! You might have to wait up to 10 minutes on the first boot, since the Dalvik has to be re-created...