If you want to trade on c-cex, after creating an account, just click "Trade" option on the left. You'll see the markets like bTC market, USD markets and so on with a list of coins listed below (available for trading in that market). Check this screenshot for the coins available for trade via BTC:
http://prntscr.com/f0cm1jThen you need to click the "Check / uncheck all" option as few coins are selected by default. Then click on the altcoin you want to trade and then save. Then you can see the current price of the altcoin after clicking "Save". For example, I clicked DASH here:
http://prntscr.com/f0cnpnThen click on the BTC amount if you want to trade in BTC. Else, select the USD amount. You'll get to see Buy DASH and Sell DASH options like in this screenshot:
http://prntscr.com/f0co9wFill in the text boxes with the amount you wish to buy or sell the altcoin. For example, "Amount to buy:" Price per DASH:" and "BTC to spend:" when I want to buy DASH. Once you click "Buy DASH" button, your trade is placed. Note: You need to have either BTC/DASH to buy or sell coins on c-cex depending on which market and coin you want to trade.
Hope this helps.