what have i won

Its a scam.  You have won a chance to have your money stolen.

how so

What did you do to win?  Enter a contest?  Put your name in a drawing?  

There are lots of scammers calling, texting, and e-mailing random people to tell them they won a prize.  They just have to pay a fee to get it.  The scammer keeps the fee and never gives the prize.  If you won a contest you didn't enter its almost certainly this type of scam.