choosing a rom is partly personal preference, but largely has to do with your phone. every rom runs differently on every phone. you just have to try them out, and see which one your phone likes best. you can find a bunch of roms in the Custom Roms section, under various developers.
the only recommendation i would make would be to not start out with MIUI. it's not a bad rom - in fact it has taken the forum by storm. but, it's relatively new, and just ported over to D1, so they are still working some things out. for a first time rommer, i would go with something a little more "mainstream". also, i would not recommend Bugless Beast right off the bat. again, it's one of the more popular roms, from one of the best developers on the forum. but, he's made some tweaks to the rom, and the install process is a little tricky. i would wait until you get a little more comfortable with the rom flashing process before trying.
other than that, all the roms on here are good: Project Elite, Liquid Frozen Yogurt, Chevy, Lithium Mod, Shadow Rom, Cyanogen, Sapphire, Ultimate Droid, etc, etc, etc....
for a rom/theme/kernel install, here is the procedure i use, and it never fails to give a clean install:
1. backup your apps with TiBu (or thru Astro)
2. create a backup/nandroid of your current rom (so you have something to fall back on if you experience any serious problems flashing a new rom)
3. boot into recovery
4. wipe data/factory reset
5. wipe cache
6. install rom zip from sdcard
7. reboot
8. IMPORTANT - let market completely download all your apps, and let it complete the process before flashing any themes, kernels, whatever
9. boot into recovery
10. flash theme (if you desire something other than stock)
11. reboot
12. boot into recovery
13. flash kernel
14. reboot
15. enjoy!