The resources tab has some of the more basic items you'll need to be successful in the game.
Trooper Frontier Keys
The cheapest way to get those, since the trooper badges are basically free.
Skill shards are very important, especially early on in the game when your characters need to pack a little more punch. Highly recommended to purchase quite often throughout your tenure in the game.
Rare souls of general status and of specific colors can quickly get you some higher level characters or sacrifice material. These become less important as you continue to play, but the sacrifice materials are always nice.
Precious Soul Fragments
Precious souls offer one of two things: Sacrificial hearts or Golden Idols. The hearts are great for sacrifice events (make sure to level them up to their highest level at the star rating you receive them at) and the golden idols can be sold for gold. These add essential resources weekly if you remember to buy these fragments.
Trooper Soul Fragments
Trooper Souls provide you with one of 4 characters, all 5 star. These should be purchased regularly to keep a steady flow of 5 star characters to your roster or marching into the sacrifice pit.