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Monica covers the main points excellently. Beyond that, if there is one specific thing I would suggest that you do to improve and remain relevant as a technical writer, it is learning to code. 

This is not about reading source code (though that is a useful skill in some jobs) nor about writing code yourself (though you won't learn unless you do), it is because everything these days runs on software. There will be code almost everything that you will document. And the reason it is useful to know how to code is that it will help you ask good questions of developers so that you don't waste their time, and so that you more readily understand their answers, and the way they think. 

Developers are critical path on every project, so you will get more respect and be more valuable if you don't waste their time. And they will be more willing to give you the time you do need if they respect you and feel you have done your homework before you ask your questions. 

You certainly don't need to know how to code to have a tech writing career, but it will open additional doors, and, beyond the basics that Monica mentions, it is one of the best things you can do to stay relevant and enhance your career. 

Unless you have a reason to choose something specific, the language to learn on today is probably Python. But don't think of it as learning Python. Think of it as learning to code using Python. And no, learning HTML is not learning to code.