To elaborate on what the above poster was saying, you shouldn't just include cards as 1 or 2-ofs in the deck. This reduces consistency. In a 60 card deck, a card you have only one copy of in has a 1/60 chance of being drawn, but a card you play 4 of has a 1/15 chance of being drawn. This means, between your opening hand and a mulligan, you have a high chance of getting a specific card in your hand on turn 1.
Additionally, your deck should always have a clear focus. The few major archtypes are as follows:
Rush: Attempts to win quickly, combining cheap, offensive allies, efficient burn and cheap weapons. This gives your deck diversity and means that control decks (see below) need to draw more specific answers to stop you.
Control: Control attempts to stall a game out, using answers to keep yourself alive until the later turns where you can seal the game quickly with game enders like The Abominable Greench, Belt of Blasting and Felsteel Reaper. They need to have answers to a wide range of threats that rush and midrange decks can throw at them. Cards that provide 2-for-1s (1 of your cards destroys 2 or more of your opponents cards) like Brok Bloodcaller, Vexmaster Nar'jo or Doshura Risestrider (as you can see, Horde are superior at this) or can solve multiple problems (Tatulla the Reclaimer, Chipper Ironbane, Puncture, Bringer of Death).
Mid-range: A deck which precariously balances control and rush. It aims to be able to control the rush decks and rush the control decks. It focuses on cards around the 4-7 cost, and heavily relies on 2-1 cards, like the aforementioned Horde choices. Another form of midrange is a deck which throws out multiple low threats early, then leads into bigger midrange allies and removal and interrupts to back them. A prime example of this type of deck is the Black Ice Fizzlefreeze deck, which relies on cheap protectors, Water Elemental and Myriam.
If you want to look up any of the cards I have listed (which I highly suggest you do if you do not know what they do), then refer to this site,
http://wowtcgdb.com/cards.aspx. It is a database containing every single card in the game.
To develop your deck and find a good starting point, look at some of the decklists covered in the more recent constructed tournaments, found here,
http://entertainment.upperdeck.com/wow/en/events/default.aspx. The World Championships were only a month or so ago, so you can look at some of the popular deck lists from there as a starting point. To get an idea of how the deck plays, look at the top 8 coverage, a turn-by-turn guide of the top 8 matches, often played by some of the best players in the world.
Once you have a decklist you like, you may wish to buy single cards for the deck, rather than sealed product in hope you get the cards you need.
http://starcitywow.com/ is a popular and well-renowned site for buying singles.
I hope this helps you, and good luck.
EDIT: Also, try to focus on cards from the newer sets, as they are normally of a higher power level than those of older sets.
And if you want my e-mail, PM me, I'd rather not post it here.
Level 0 Judge (Because apparently under 16 year olds can't get Judge certificates =/)
The Official WoW TCG Forums Cross-Dressing Englishman From Hawaii