Get far enough in that you have like 10 or so "E" and you should be able to hit "Select" to pull up the powerup menu. While on this menu, move your ship over the powerup you want to enable. "Sheeld" (lol) is the one you want, and it'll make it so any time you get hit (would have died otherwise), it will instead take away 2 "E". A VERY worthwhile trade, if you're not great at dodging in general.