so where to exchange legally?
The biggest exchange, if you're interested in day trading, is Mt. Gox:
https://mtgox.com.
They've been tangled up in some legal trouble recently. A smaller, but currently safer, exchange is CampBX:
https://campbx.com/main.php?r=2D63QDP2hvqIf you want to buy Bitcoins, but aren't specifically interested in day trading, you can also buy through BitInstant (
https://www.bitinstant.com/) or Coinbase (
https://coinbase.com/?r=51a5b0d3867ba8db8e00000c).
If you're not sure what you're looking for, I have an article that's aimed at getting started from scratch, starting with setting up a wallet:
http://www.bit-buddy.com/bitcoin-wallet.html.