How to register at Ghash.io
Register and transfer a small amount into your balance. Keep a close eye on the price as the mining on cex.io is no more profitable than buying your own hardware. The thing that makes it different is that you can sell at the market price instantly. So just make sure that you sell for the same as you bought it (roughly) and that the difference if lower is less of a loss than your gain mining.
Been spending some time getting my head into it, and its mining but a bit more - make sure you understand that 'bit more' before investing more than 20Ghs.
CEX.io is my link if you need it, and happy to help you through the learning process via pm if you need it.