To make a deposit, go to "Wallet" through the main menu or deposit button.
Select the cryptocurrency you want to deposit and click on the "Deposit" button.
Select one of the payment channels/networks.
Send funds from your wallet to the displayed address. As simple as that!
The deposit logic is the same for other cryptocurrencies.
To start trading P2P contracts, move funds from your wallet to one of your trading accounts using the internal transfer ('Transfer' button).