HTM L is hardly a must (and hardly even a language) unless you want to create web pages, and even then there are a lot of tools that will create HTML for you so that you don't need to do the tedious parts yourself.
What you should learn depends on what you want to do. The C language might be a good place to statr if you are interested in learning programming, although there are many other choices. You could start even simpler with something like BASIC but you will find it more limiting. You could jump right into assembly, although that can be pretty technical. And there are many many other choices, the choice is generally dependent on what you want to do, so give us more input to help us better guide you.