You need some bitcoins to use as a security deposit.
You can also only buy ETH with bitcoins so you will need your trade amount as well as with all the fees.
You need to setup an altcoin account in the app by going to “Accounts”.
Then you just need to select the ETH altcoin in the app and click on “I want to sell BTC” button.
That will display you offers for buying ETH with BTC and if there are none that you seem interested in, you can create an offer your self by clicking the appropriate button.
That is more or less the overview, but I am sure that you will be able to figure it out, it is quite intuitive.