You answer:

Open the page of alexa.com with different plans, click the tab, choose the one you consider appropriate (you can take the cheapest one), and then follow the guide. Take into account that as soon as you install the traffic counter web traffic generator Sigma , you have to wait for 3 weeks till you can change the rank from estimated to a certified one. If you don’t do that by default alexa.com shows an estimated rank even if you have it active and certified.