Just keep posting and you will see your tbt increase (your amount is under your avatar), it is the forum currency. Igb is just in game currency.
You can do all sorts with tbt, even selling it on to buy igb (or the other way around),buy collectibles (which are the little picture some people have in their sidebar). The majority of people on the trading plaza will ask for tbt in return for villagers too.
Oh and the current exchange rate for tbt to igb is 20mil igb = 100tbt