Once you've settled on an idea, it's time to secure a domain name.
You'll want to purchase a domain name from a reputable company (they're called registrars) that has been around for a while. You don't want to register with a company that may go out of business.
Otherwise, there is no performance benefit of choosing one registrar over another. Some good registrar options are (I've included affiliate links, please use them if you could as I get a little bonus if you do):
I didn't list Godaddy.com intentionally. I was a customer for a long time, but I don't think their user-interfaces are very friendly. They appear to be geared more toward enticing you to purchase add-ons than to help you manage your domain.