What to do before root and flash my phone?

1. You will need to root first. Without rooting your device, you will not have access to certain parts of the system, and flashing a ROM might crash the device (most ROMs need root). You can root without erasing or resetting anything, so you can continue with #2 and back stuff up before you flash your ROM.

2. Configuration you will probably lose, but apps and app data can be backed up through Titanium Backup. The SD card will not be affected by rooting, resetting, or flashing ROMs.

3. Going back to the stock ROM is more difficult, unless you're okay with still being rooted. After root, boot into Recovery and choose Backup and Restore->Backup or similar (different on different recovery versions) to do a NANDROID backup. This will store an image of your original ROM, and is a one step restore to normal.

3a. Unrooting is entirely a more complex process and will involve re-installing your stock ROM (if you can find it online) and deleting everything on your device. NOTE: again, SD card will be okay, and apps can be restored with Titanium backup.

1. You should flash the ROM first, unless you need root in order to flash it. (You can flash via Odin without root, but you need ROM Manager, which needs root, or something to flash if you don't use Odin). When you flash a ROM that's rooted, your device will be rooted; if the ROM is not rooted, the device will not be rooted.

2. You need root to make a full backup. See How to backup an Android device?

3. If you root first, you can install a custom recovery (e.g. ClockworkMOD, via ROM Manager) that will let you create a Nandroid backup of the full ROM. That said, with a Samsung device you don't need to do this. You can use a tool called Odin to reflash the original firmware as long as you've found it for download somewhere.

If you need more details on anything I've mentioned, I suggest searching first -- we have lots of questions on rooting, backups, Nandroid / Clockwork / ROM Manager, and Odin. See also How do I root my Android device?, which has some info on considerations and things you can do before rooting.