Dust is used to improve your cards. Wait until you lose your first single player mission to first build a worthwhile deck and get a sense of which cards will be better than the others. Because the price is always the same for the upgrades, begin with your fastest cards first. They will gain more health and last longer in battle, allowing your slower cards to have a chance to be effective. Spend Dust to upgrade your hero as well, after all important cards have been improved.