Dust - you can get dust from the arena rewards or from disenchanting cards (which turns a card into dust). You don't get a lot of dust per card for the common cards (the stone in the middle of the card shows it's rarity - common white, rare blue, epic purple, legendary Orange) but legendaries give 400 dust. Gold cards give 2* their dust value so a gold legendary gives 800.
So, you have some dust. You want to create a card. Goto your decks > crafting mode (top right), find the card you want to make. The ones in faded blue mean you don't have any of them yet, the ones in colour you already have at least one (look underneath the card - you might already have 2x). Once you find the card to make you click it and the button to create it. Depending on it's rarity it will require a different amount of dust to create. Commons cost 40 dust, legendaries cost 1600 to make.
This same interface also allows you to disenchant cards - it will be the other button (there's one to create, one to dienchant). You'll see how much dust you will receive in green or how much it will cost you to create in red.
Check this page out for the other details:
http://hearthstone.gamepedia.com/Rarity
You can not disenchant cards given to you by the game for levelling. There are considered "basic" and can't be disenchanted into dust (eg soulfire).
Game goes fully open in a few weeks to quote a blizzard employee:
We currently plan to invite all remaining opt-ins via direct account flagging and it will be done as one large wave. This invite wave has not yet happened, as we are still compiling the full list of accounts to be flagged. A notice will be sent once the wave happens.