Just go read freaking recipes. http://fallout.wikia.com/wiki/List_of_Recipes_%28Fallout:_New_Vegas%29
You want to keep everything that makes decent food, improves drugs, heals limbs, fixes weapons. Also learn what you need to repair your stuff, with Jury Rigging it's way easier, Metal Armor fixes Power Armor, Raider Armor fixes Combat Armor, all rifles can be fixed between each other too. Also keep everything that weights 0 or close to 0 but is an ingredient or has value.
In general only sell cigarettes, armors and weapons you don't need for fixing stuff, rest dump in Novac in case it's needed one day.

Excess ammo can be broken up and later converted into what you need.