The one thing you will learn very quickly about Marvel Strike Force is that you will need plenty of resources. Your best source for getting them is to complete the Daily Objectives where you can earn rewards such as gold, materials, and Commander XP points which will help you level up your characters. Daily Objectives reset every 24 hours.
At the start of the game the resources you earn from completing the Daily Objectives will enable you to easily advance through the early levels. Even in the later stages of the game you should not neglect the Daily Objectives as the rewards you can earn from them will help you at the advanced levels.