Newby question.
To make an online game, it is necessary to build a server and client with Visual Studio 2015.
I am not going to change unreal engine code.
I was wondering if I really need to make a fork, or can I just clone the unreal engine itself (In github desktop)
It is not necessary to create a fork of the GitHub source in order to build the Engine. Downloading the zipped source code for the version you want to use should be sufficient (I have a collection of zipped source code for each Engine version on my secondary hard drive). Having a fork of the Engine can make it easier to upgrade the Engine when newer versions are released, but if you don’t intend to upgrade or modify the Engine source code, then you don’t need it.