You can't. ONLY one user can use the PC at a time (physically).
1 Computer runs one OS ONLY at a time.
Virtual OSes, are for using a second OS inside application window for some other tasks (i.e. running MacOS in a VirtualBox to test compatibility with the application you just made for Windows 8).
A server can HOST multiple clients to perform multiple tasks, but each client computing device (often called a Dumb/Smart terminal) is still one PHYSICAL computing device for ONE Physical person to use.
Lastly the technical aspects to any of these solutions are not a simple "teach me in 5 easy steps" and are best and often handled by PROFESSIONALS since there is a MYRIAD of issues that can come up with any of these functions you need to...