To start, you will need an energy weapon, either Laser or Plasma based. Note that Plasma based explosives don't count. You can get a Laser pistol right at the start of the game from Doc Mitchel, but after that they are hard to find. Fiends have energy weaponry in large supply for some reason, so they're a good target, as do the Brotherhood of Steel. The Silver Rush sells energy weapons from its almost unlimited supply, but are quite pricey overall. Also don't forget that traveling merchants can repair any weapon for a price, even if you don't have a second weapon to cannibalize for parts.