What is the first opening screen on computer called?

That depends on which one you are referring to. It could either be called the "splash screen" or the "welcome screen". The "splash screen" is the screen that shows the progress of the operating system loading. For example, in Windows XP, the splash screen shows the Windows XP logo, along with a progress bar at the bottom. The "welcome screen" appears immediately following the splash screen, and is what allows you to log into your computer after the operating system finishes booting up.