There are many ways to learn to code these days. From coding bootcamps like Flatiron School to online courses to totally free beginner resources. The best way to learn to code depends on many factors, but the most important one is you. See what course is best for you.
You might also want an addon for Thunderbird to let you choose the Template/Stationary more easily: https://addons.mozilla.org/en-US/thunderbird/addon/stationery/
Beyond that, and most importantly, all of your images and hyperlinks must be available publicly and it is best to use absolute URL's. When I created my own, back in 2002, I had to go in with Wordpad and change all of the HREF's to http://www.computer-help.net/Images/<graphic-filename> for it to work.