You can earn Steel by completing certain actions in-game, for example by finishing matches and tutorials, completing orders and sometimes by locating and destroying breakables in story missions. You can also buy Steel Packs of varying amounts from the in-game store.
These are the available packs:
10,000 Steel + 1,000 Bonus = 11,000 Steel
20,000 Steel + 5,000 Bonus = 25,000 Steel
50,000 Steel + 15,000 Bonus = 65,000 Steel
100,000 Steel + 50,000 Bonus = 150,000 Steel
To find the in-game store, simply open the game menu and navigate to the tab Store.