Senn, it's currently in closed Beta. That means they're only letting people on who ask nicely or can get an invite through something like shattered crystal (which is free).
After release, everything should be available free, but if you buy the "premium service" you get experience and silver money at double the rate.
You earn silver from fights, for doing damage and spotting enemies and capturing flags.
During Beta, they're giving away "gold" at the rate of 150 per day. It costs 2500 gold for a month's premium (or 250 per day).
After go-live I think gold will be purchasable.
Some of the higher level tanks are so expensive to fix that you can't really make money with them if you lose them in a fight (which for me happens about 75% of the time) unless (and this is the important bit) you're on a premium account.
So, without premium, you can still drive a king tiger, but everytime you get exploded, you may have to grind several fights in a T-34 or other cheap ship, I mean tank, until you can afford to fix your king tiger.
Oh, and apparently the Russian heavy tanks are better than the German ones (partly because the Russian heavies go through to the 1950s and the German ones stop around 1945 for some reason)