Buying the right domain name matters. If you get the wrong one, you could find yourself with a lot of problems - like terrible ranking where you have to fight to get Google to rank your website. That's not good and it's something that can easily be prevented.
Stick with a brand name and NOT a keyword for your domain - that is most important. For example, "Nike" is a brand name. It doesn't mean anything except for the items that Nike sells. It's not like "BestWebsiteDesigner.com" or something silly like that. Do not buy a domain name that is a phrase someone would type into a search engine. That would be a monumental mistakes.
If you can, but a domain name that's a new word. Nike is a good example and so is Webstix. That's a new word. The benefit is that it's super easy to track it online. This is probably the best advice I can give you on buying a domain - go with a brand name that is a new word... you'll thank me later.
Some domain name buying basics:
Get more tips here:
21 Tips on How to Choose the Best Domain Name for Your Website (tonyherman.com)