You have 2 stages on your turn. Stage 1, you sacrifice a card (or click Skip if you don't want to). Stage 2, you use your cards. In stage 1, you have to either sacrifice or click skip. You can't do anything else at that time.
After you do one of those two things, you can then lay down cards and/or attack. To lay down a card, you must have enough resources. Your resources are your sacrificed cards. Each card has a number at the top left that tells you how many resources are required to play the card. If you don't have enough, you can't play it. If you play a card, it uses the resources for that turn, so you can't use the same resources to play multiple cards. However, if you have something like 4 resources and two cards that cost 2 resources, you can play both on the same turn.
As far as attacking goes, you cannot attack with a unit card on the same turn as you lay it down unless you have a card played that allows it. So if your card is "grayed" out, you can't attack that turn. Any unit cards that are not "grayed" out can attack as long as there are not cards played that prevent attacking. Some cards prevent you or your enemy (or both) from attacking for a certain number of turns. It may prevent attacking allies (the units) and still allow attacking the hero, or it may prevent all attacks. You can still use "spell" type cards to attack even if your units cannot attack. To attack with a unit if it's not "grayed" out and isn't blocked from attacking by some card, just click on the unit and click Attack. Then click the target (enemy unit or hero) and click Target. Repeat with all units that you can use and then end your turn.
Be aware that if you do not kill a unit with one attack, it will retaliate and damage or kill your unit.
Good luck and enjoy the game. As I mentioned in another post, my only issue with it is the difficulty if you choose a poor hero and that I can't manage to get the "free" crystals by downloading games through the ads.
*HINT: When choosing a hero to start the game with, avoid heroes who say they get a bonus to attack. That isn't much good in the beginning of the game because they can't attack until you get a weapon card, which you won't start out with. By the time you get a weapon card, you should be able to get some more hero cards if you want the attack bonus. You'll be better off getting a hero who can damage/kill units every few turns or bring units back from the dead.
