There will be missions that will require upgraded cards. Do this only that at the beginning, since you will not know which cards are good and how to build a useful deck. Only upgrade the cards by the minimum required to complete the quests. Use your basic cards until you are forced to improve them to progress. By this time you will obtain useful cards that are worth upgrading. When upgrading, start with your best cards first. This is usually the rarest cards as determine by the number of stars. If you get a better card, you can use it to replace one that was not upgraded and have extra upgrade materials. Consider working on two cards at a time. Pay attention to elements; use the element booster on the same element cards for maximum experience points.