Use Domain Search - from the homepage or the client area. Type the name, and the results show whether it is free and what it costs per year, along with suggestions for other endings.
Press Register on the one you want, and it goes to the cart like any product. After payment the domain is registered to you and appears under Domains.
From its page you manage everything: nameservers, DNS records, the registrar lock, ID protection and renewal. Auto-renew is worth turning on for any domain you care about - an expired domain takes your website and email down with it, and getting one back after expiry is slow and sometimes expensive.