Hello, I know a bit about wow tgc rules and overall. But what's puzzling me is to.. which expansion should I choose to create my first deck? I know there are many different starters/boosters that were released so far. At the moment I have Onyxia raid deck ,62 cards that came with Wrath of the Lich King collectors edition. and two Drums of War  boosters. I also bought two starters of Heroes of Azeroth which I bought recently for low price on ebay. Is it a good start? What's the actual difference betwen all those series that came out so far?

The difference between the sets is mostly a slight power creep which makes the latest sets a slightly better aiming point. There are some very good cards in the older sets but it's probably better to pick up the ones you want as singles
If you want a core to build around, I'd recommend the Drums of War starter set. This contains 2 fixed pre-constructed decks (one Mage & one Hunter)

Thank you for your reply. So besides the slight differences in power of the cards it doesnt matter which series I choose? I know about the raid decks and how they work more or less. But recently I found out about the hunt for illidian cards and It seems there are some new mechanics introduced there. Are these cards meant to be used separately in similar manner as raid decks? Are there any other series that somehow differs in this way? Actually what I want to know is if i can combine the cards from any of the series freely with each other?

Ps. I'm sorry for any language bugs in my text as my english is still very rough.

I think the only cards beyond the Raid ones that can't go into decks are the Arena Grand Melee ones (although the new 'preparation' cards from that can be)
There's new mechanics but I think the only restrictive labels are Traitor/Loyal & Aldor/Scryer - basically pick one of the sides, you can't use both in the same deck.

Beyond those slight tweaks- mix & match to your hearts content

There are many types of formats.  For the Constructed Format cards from each and every set can be used (so you can use Hunt, Heroes, Drums etc).  There is also Block Constructed.  A Block is 3 sets, so Heroes of Azeroth, Through the Dark Portal and Fires of Outland is one block.  When you are playing among friends, Constructed is generally used as it is the default format.

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I think that you Leonard have as much of a chance of having a sexual relationship with Penny as the Hubble telescope does of discovering at the center of every black hole is a little man with a flashlight searching for a circuit breaker.

I see. I guess that concludes my doubts. Thank you guys :)

some other stuff you might run across and arent sure of. all heroes that arent traitors are counted as loyal heroes (they dont have it printed on them tho). and the demon heroes in hunt for illidan each have unique deck constructed rules they must follow, idk where they are posted other than http://www.wowtcgdb.com/cards.aspx

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"We must see what further cards we have in our hands and play them with decision." - Sir Arthur Conan Doyle