You can get Coins by killing zombies and rescuing the civilians. For each zombie you kill you will earn 10 Coins at the end of the game, and for each of the civilians that you rescue, you earn 50 Coins. Accidentally killing a civilian causes you to lose 50 Coins at the end of the game. To get Crystals you have to buy them with real money.
Free Coins
If you manage to complete a level in under 9 minutes you will be rewarded with a 'Time Bonus' which is almost 600 free Coins.