You must have downloaded an app that is actually a bitcoin miner disguised as an app
It uses your whole computer power and internetconnectiom
FTFY.
Also miners do not require that much bandwith. Without any knowledge about which "app" or programm OP downloaded and uses we can only speculate. So its probably best to wait for an answer first. I suspect its bitcoin core/qt and is just syncing the blockchain.
oh sorry yes the normal bitcoin qt from here and then i want to sell a coin as i am overdrawn in my bank
It will need some time to catch up on the latest blockchain events aka sync the blockchain. There should be a greenish bar at the bottom telling you how much time your version of the blockchain is behind the rest of the network. You can also see how much bandwidth its using via help -> console -> traffic (the actual words might be different, cant check that right now).
You have several options:
slow and safe: just wait, it will be done syncing eventually. How long exactly depends on your network speed, how many blocks you are missing and some other less significant factors.
faster and slightly more risky: you can export the private keys you need to spend the coins and reimport them into another slim (e.g. multibit) or online wallet. The risk depends on your knowledge regarding wallets and how to securly handle private keys.
fastest and most risky version: send someone your wallet.dat and password along with your deposit address for the exchange and let them do the transaction. Speed and risky depends on who you send it to. Someone you know personally and trust that has a fully synced bitcoin core/qt would be best.
IMHO: go with option one
-Sho
