Simply use the slider to indicate how much hashrate (processing power) you wish to purchase. Each TH/s (terahash) or 1000 GH/s (gigahash) costs $220 USD.
You can use the calculator below to see how much return you will get for the amount of TH/s you purchase.
As a rough guide using a BTC/USD price of $12,000 - 1 TH/s will return $494.37 USD after 1 year. If the Bitcoin price went up to BTC/USD $18,000 - 1TH/s would return $741.56 USD after 1 year. So your returns are based on the value of Bitcoin.