Concentrate on C / C++ as that can be used on any platform - Windows, Linux, Mac, iOS, Android etc...
Many other "mainstream" languages like PHP and Java / Javascript have similarities to C and you will be able to adapt quite easily if you ever need to; however they are more special purpose than general purpose like C.
For app programming, grab copies of "Livecode" and "Marmalade".
Both can be used to create cross-platform apps; Livecode uses it's own language which is compiled to native code for whatever device or machine you select as the build target & Marmalade can be used with C++ for both Android and iOS.
Each has a massive number of examples of code, all the way up to complete game sources, so you can experiment and learn how they work.
Livecode allows you to edit things while your program is running in it's simulator and instantly see the results of changes.
They are also both free for personal use.
https://www.madewithmarmalade.com/
http://livecode.com/
ps. Note that you cannot submit apps to the Apple app store with both an Apple Mac and an Apple Developer account. Look at second-hand Mac Minis on ebay, they are the cheapest way of getting a functioning Mac.
A 2012 or later model should be fine.