In the crafting menu, you spend your hard earned coins in order to unlock new recipes for you to craft when in missions, but also upgrade existing ones to give you better results. My advice is to focus on the healing and damage dealing items first, the slowly work your way up unlocking and upgrading new items. You don't really have much of an option but work on all of them and focus on the cheaper first because even though you can unlock a better armor, for example, you might not have enough materials to craft it in the game, meaning that it's still the one with lower stats that you will craft and use (since it's better than nothing).