Welcome to the site, dmytron

The current state of cross-platform development (writing code that runs on windows, mac, linux) is still in flux and not yet settled, so there is no one answer that is going to be completely satisfactory.
Recent developments in .NET are exciting in terms of the possibility that it will eventually become a serious option for cross platform development. I don't think it's quite there yet.
Java has proven itself to be a good option for cross platform DESKTOP coding, and it is the preferred choice for Android mobile coding. That leaves out Apple's iPhone/iPad of course.
If you aren't interested so much in Graphical User Interface stuff and mobile stuff, you also have some other options like Python.
I'm sorry that I don't have a very specific answer for you, but I guess my best answer goes something like this:
*DO* try to learn another language and ide. Java would be a good choice and so would a .NET language like C#. Start slowly and don't bite off more than you can chew.
The modern IDEs are insanely over-complicated and full of so many options and features and tools and stuff, that it can be completely overwhelming. I think we're going to see a backlash against this one day -- because in the early stages of learning it makes everything more confusing and overwhelming. Get yourself a good book and/or walk through some tutorials and just learn one thing at a time.