Deck construction:Once you pick the hero you are using, try to pick cards that synergize well with your hero power and the hero cards that you like. For example, as a Shaman, you can create many totems on the board with 0/2 so they do no damage but help in other ways. If you can use cards like Bloodlust that give all your minions +3 attack this turn, then those totems are suddenly a huge problem for your opponent.
Card draw is important and useful in almost every deck. Since you only draw one card per turn, other cards that draw cards are powerful. I once built a deck full of 1 and 2 mana creatures thinking i could overwhelm my opponent early on and just keep playing a ton of minions. By turn 6 though I'd have 6 mana but only one 2 mana card in my hand! I wasted a bunch of mana by not having enough higher cost cards to play. This deck could have worked if some of my cards drew other cards.
When creating a deck, its important to have a good mana curve. This means that you should have a good distribution of 1, 2, 3, 4, 5, 6, 7+ mana cards. On the deck building screen it will show your current mana distribution when you hover over your hero in the top right corner. Generally its best to have a few 1 or 2s, many 3-5s, and a few 6+ cards. It depends on your deck and your play style though. If you are a Mage with the ability to clear most enemy cards early, then holding out until late game and using higher cost cards could be an effective strategy.
In game:
Having a board advantage or a hand advantage is typically better than a health advantage. The goal in this game is to bring your opponents health to 0, so it seems obvious that having more health than your opponent means you have an advantage. That's often not true though. A board advantage means you have more or stronger minions on the board than your opponent. If you maintain a board advantage this will very quickly lead to your opponents death even if he had more health than you. Its easy to lose a board advantage though if your opponent has a hand advantage. By having a lot of cards in your hand you have more options. If you have 4 minions on the board and 1 card in your hand and your opponent has 1 card on the board but 8 cards in his hand, your opponent has a lot of ways to wreck your board and kill you.
It's usually better to clear minions first rather than attack the enemy hero. This is because leaving creatures on the board gives your opponent more opportunities to buff them. A 1/1 minion in the right deck can quickly get out of hand and become a 5/5 threat. By missing opportunities to clear your opponents minions, you are giving them more options for their next turn.
Don't be afraid to suicide your minions for the right reasons. It's rarely good to skip attacking with your minions, and by being the aggressor you can decide what trades are best for you. If you take the 'wait and see' approach, your opponent gets to decide the pace of the game. Although, to contrast this, just because you can play a card doesn't mean you should. Even if you have to waste mana by not doing anything other than using a hero power for a turn, its better than wasting cards. A good example is the spell Polymorph which turns a minion into a 1/1 sheep. You may be tempted to use it on the enemy 4/4, but that spell would be much more valuable when used against an enemy 7/5. So if you can sacrifice two of your minions to clear the 4/4 and save your Polymorph for a time when you really need it, then that's the best choice.
Be aware of the enemy hero's potential. Against a Mage be careful playing all your minions at once because their spells can clear your whole board. And against a Priest, if you think you're so smart playing your biggest creature on turn 8 with nothing to support him, then you're in for a surprise when he gets mind controlled and the Priest kills you with it. The capabilities of different heroes you'll mostly need to learn through playing though.
I hope these tips were helpful. Let me know if you disagree or have any questions.
