Best coding skills to pick up

I’ve worked a variety of jobs in IT for the last 20 years in systems support. 20 years is long enough to wish for a change, and after having classes in various programming languages over the years (which I loved), was considering trying to re-enter IT on the developer side of the house.

What are the best current technologies to learn in order to make yourself marketable?

Also, wondering if it’s possible to get side jobs at home as a way to gain skills and still keep the regular job until ready to transition?

Any advice would be greatly appreciated. .