PerterP wrote:I have thousands of questions on developing iOS applications.
This is normal. :)
When embarking on something like this, there is a *lot* you don't know, even if you know how to code. A big part of the developer's journey is learning what they need to know, and solving problems. There is no easy fix for this, you just keep going and learning until you get to a place where you have the puzzle pieces solved.
PerterP wrote:The first one I need to solve is, I need to know the best Mac OS Version to run the X-Code. I heard some news about Mac OS X El Capitan as the best Mac OS Version to develop iOS applications.
*Best* is rather simplistic, and will vary depending on who you ask. If you want to sell your products in the App Store, then there are specific requirements you'll need to understand and follow. Otherwise, it might not matter too much. Of course, what platforms/frameworks your code links to in order to do it's job are different - you can use X-Code on Sierra for example to compile code that links to earlier OS frameworks so your software will work on earlier systems - but again there are some caveats here (which you'll learn as you encounter them).
I short, commercial Mac/iOS development is mostly done on fairly recent systems if you want to keep things compatible, but some of these things are less important that just *starting*, learning how to code, design, how the Mac development environment works, and so on - if you get to a place where you need a later version of XCode, then you sort that problem out then.
For iOS development, using something fairly recent to avoid running into odd problems with old stuff, would be my advice, but I don't think it matters too much at this stage as long as you're not running super old stuff.