Your first mod should be an intake and tuner. Either C&L or JLT with either an SCT or Predator tuner. Buy from either BamaChips or Brenspeed. After that should be gears.
Here is a great write-up on installing a DS.How difficult is it to install by yourself? I'm not what you would call car savy (I know the basics but not as much as most people here it seems). And what are you looking to sell your for?