http://www.nexusmods.com/newvegas/ (host most of mods created for New Vegas on the web)
Two indicators of a mod's quality are:
The number of endorsements
The number of downloads
http://www.nexusmods.com/newvegas/mods/topalltime/?adult=2
The above page list the most endorsed files of all time for NV, so they are all guaranteed to be high quality. Whether they are to your taste, that's dependent on the player (you).
If you're new, try to avoid mods that overhaul the entire game in massive ways. Start with a texture pack, re-textured or new clothes, then move up to something a bit move complex. Mods that mess with the factions should probably be the last things you try. If you're experienced, go wild.
I don't have NV for PC, but people who created "famous" mods for FO3 probably creates the same mods with more new modifications for NV as well. A few names to look out while searching for mods are:
NMC's texture pack (better texture for most if not everything)
Fellout (change the atmosphere of NV, make it brighter, more vibrant, I don't know as I don't have it)
Martigen's Mutant Mod (changes the creatures of the Wasteland, randomizes their sizes, one of the best for F3)
EVE (don't know the specific for this)
If you can't decide which mods to try because there are so many of them, I can take a look at some of them and give a recommendation.
Any questions, don't hesitate to ask.
