You will need an alternate start mod like this one: http://newvegasnexus.com/downloads/file.php?id=37390
That will break the main quest and let you start wherever you place the appropriate marker.
Then you'll just need to go into the GECK and start deleting all the stuff you don't want. I would stay away from deleting the actual quests/scripts until you have a better idea what you are doing but you can start removing NPC's from areas etc.
Once we have a full version of the NewVegasEdit, NewVegasModManager and a few other tools myself and another modder are hoping to clean out the esm so that modders have a blank slate to start with.