Most Linux versions are a collection of software, known as a 'distro'. A distro represents a collection of usually freely available software. The requirements are the kernel, system utilities, some server services if desired, and a desktop or 2. You should also have any additional preferential utilities, the creation or use of an installer program, and, finally, a ISO image that boots up, and presents a menu that starts the installation. You can also build a 'live' distribution of all this software and create a 'LiveCD' version with no installation necessary. This boots and uses the DVD/CD as the installed disk.
Understand that most Linux versions are free to obtain and free to redistribute. Decide on which Linux distro to start with, such as Ubuntu, Fedora or Debian. Linux has a Graphical Interface, but most of this requires you to type commands and or compile some programs if starting with source code. If you want to use Linux, try suggest Ubuntu, Linux Mint or another user-friendly distro.
Install one of the distros on your computer.
Decide whether to create a distro. There are 3 types of distros:
Remaster (known as redistribution or remix,too). This only requires the change of a wallpaper, installation of some software, and publication of an ISO image. One of famous remasters is Linux Mint.
The second type of distro is Fork. Fork is very similar to a remaster, but it has its own repositories, kernel configurations and other changes. A famous fork is Ubuntu.
The final type of a distro is an independent one. Independent distros are compiled from scratch by builder(s). A famous example of an independent distro is Debian GNU/Linux.
Choose GUI, Default software, etc. You need to choose a desktop environment for your GNU/Linux distro. There are some famous desktop environments, Like GNOME,KDE,XFCE or LXDE.
Also, you can use Window Managers as default GUI (WM's are simpler than DEs and they're lighter and suitable for old and weak computers.) Some famous window managers are TWM,FVWM, IceWM, OpenBox or FluxBox. For default software, you need to choose a web browser like Firefox, a media player like VLC and an office suite like LibreOffice. You should also choose other softwares that you need.
Choose a good name for your distro. It can be your name, your last name, your city name, your favorite music band's, artist's, actor's (or actress') name, or a mix of some names. For example, Debian is a mix of Debra-Ian.)
Design a website for your own distro. You need to advertise your own distro. You can use SourceForge.net or other OpenSource project hosting websites. You can also can use free hosting services, or you can pay money to have a good website.
Submit your distro on DistroWatch. DistroWatch is a directory of many Linux distributions.