It is very common that the domain name you want to buy has been taken by someone. The registration of a domain follows the first come first serve basis. If you realise that the name you wish to register has been taken, you can try to buy a different name or select a unique domain extension. 

Get domain name ideas

It is not easy to think of a domain name that is catchy and yet available for registration (unless someone is selling it for a reasonable price), you can try to get some ideas on this website: http://www.leandomainsearch.com 

Try a unique extension

There are many new extension recently introduced by ICANN such as .site, .rocks, .online, .me, .clothing, .sale, .space, .rent, .love, .style, .club, .fashion, .website, .london, .paris, .world, .market, .deals and many more. It is not necessary to always choose a domain extension that is conventional such as .com, .net, .co.uk, .biz and so on. It is better to choose an extension that closely match the business you are doing such as .fashion, .deals and so on.