After you find your domain name for your Joomla website, it is time to register it as your own. If you visit one of these sites and set up an account, you can then claim the domain name.
I have a few suggestions.
If you did find a .com that you liked, definitely purchase the .net and .org if they are available. If you have your heart set on a name and it is available as a .net, take some time to research the company using the .com domain. The goal is to make sure your visitors are going to your website. If a potential customer visits the .com by accident, will they know that it is a difference company? If you have your heart set on a name and it is available as a .net, take some time to research the company using the .com domain. It can save you from trouble in the future.
Just recently, a new top level domain became available, the .co. It is hard to tell if this is something that will take off, but if you are using the .com and the .net, it may be a good idea to grab it. They are somewhat more expensive ($20 - $30/yr instead of $10/yr on average), but I don't want to wake up one day and see someone built a website on www.joomladesigner.co.