For the most part, you can pick whichever TLD you prefer, though a few have restrictions -- you can't buy a .edu domain unless you run an accredited post-secondary school. Many, but not all, country-specific domains (.us, .uk, .jp) are restricted to citizens of the country in question. Unrestricted country domains are often used for "domain hacks," where the extension becomes part of the title, such as YouTube's alternate address, "youtu.be."
Is your domain name portable if you change service providers? I’m starting a non-profit and I want to hold the name, but eventually I will fire someone to do the website development and want them to be able to use the platform they think they can work most effectively in. If I were to register with GoDaddy for services and get a free domain name… do I lose it if I switch to another web-hosting provider?
The longer your domain name is, the harder it is for people to remember it and the more chance you have of someone misspelling one of the words. Most good single word domain names are long gone, but you can still avoid long domain names by getting a little creative. If you have a single word you really like that is not available, try adding an adjective or verb in front of it and seeing if those variations are available. Think of your domain name as part of your brand, and make sure it matches how you want people to think of you.
Absolutely. Owning 'yourname.com' (as well as related domains such as yourname.tech or yourname.me) is a great way to brand yourself and retain control over your name's online presence. With a personal domain name, you can set up a portfolio, blog, or hobby site that’s associated with your own personal life. You can also set up a custom email address like firstname.lastname@example.org allowing your visitors an easy and memorable way to reach you. So even if you have no immediate use for yourname.com, it's wise to register a personal domain to ensure that you (and not some stranger) control your name online.