Coins can be be earned by killing zombies and/or rescuing the civilians. For each zombie you kill, you earn 10 coins at the end of the game, and for each of the civilians that you rescue, you earn 50 coins. However killing a civilian causes you to lose 50 coins at the end of the game.
At the end of a level, you also get a time bonus. If you make it through 9 minutes you’ll get almost 600 free coins.
Also, the more zombies you kill at the same time the more coins you'll get.
To get tCrystals you'll have to buy them with real money.