Just signed up for the web interface. Extremely confused. Please help!

1) How do I secure the webwallet? I was only provided with a pw and no private keys; how do I get the private keys?
2) 2) How do I use my BTC to purchase other coins? Does the web interface need some kind of connection with a local wallet like Parity?
3) There's a deposit feature for BTC. If this is a decentralized exchange, why do I need to deposit here?