So you've picked an open-source license from choosealicense.com, huh? FANTASTIC!
Now comes the fun part: adding that license to all your public repositories. This website will do just that. It'll give you a list of your public repositories that don't have a license file. You'll pick which license you want, and it'll add the new license file to your repos. SIMPLE!
This site is not owned by or affiliated with GitHub. But I work there, and I'm using the API to add each new license file. You'll be asked to authenticate this app for your public repositories on the next page.