Hi Ahmed, and welcome to the site!
#1. If you need (paid) technical support to help you, I'd guess that Red Hat, Ubuntu, or SUSE might be your best options. If you want free server distros, and you are responsible for everything, then Debian and CentOS are also good choices. There is no "best distro"... you have to determine which you are most comfortable with and can understand the administration duties you need. Only time and testing will make that clear to you. However, just to host a website and provide Android app downloads, you don't really need to run your own server... you can buy a domain name and rent server space from web hosting companies who specialize in this. Then, you don't need to learn server administration... you only need to learn how to use the web-based interface to manage settings, and maybe a FTP client to transfer your files to build your web pages. Many of these hosting services are fairly reasonably priced.
#2. Maybe I'm not quite understanding your distinction here... but a "server" is a computer (hardware), usually specially designed to be a server, but a simple desktop PC can also work in some cases. It can run Windows, or Linux, or whatever. HP builds servers, and IBM, and Dell, and so on. They can be very expensive new, but I imagine good deals can be found for used equipment, just like with anything in electronics. I'm sure there are instruction manuals for these devices. Linux server distros (software) often have manuals too, although maybe not every distro. Here are a few that you can look at to get an idea of what's involved:
Red Hat,
CentOS (similar to Red Hat),
Debian, and
Ubuntu.
Terminal commands: It is common for server distros to not include a graphical user interface (GUI).... which means you may definitely need a firm grasp of the command line for system administration and maintenance. But it all depends on what you decide to do... if you have a server in your home or office so you can access it directly, you might install a GUI to simplify your tasks. (But really, the terminal is often the simple way!
)
Good luck!
