The diamonds you start off with will be put to best use if you upgrade your magic academy (as soon as you get it ... wait to spend them until you do). Upgrade as far as you can before you do anything else. Then, if you need spell fragments for a quest you can disenchant. Whatever you do, make sure you don't disenchant things that you may need later, like troop instants, coin and supply instants, portal profits, or even relics. If you're positive you won't need it, you can disenchant buildings, but as I said, hold off on that too. Your inventory will hold them, so no worries about reaching a max on inventory.