How do you use computer without internet?

While you may be taking internet access for granted and consider it unimaginable to use a computer without it, the truth is that an internet connetion is not - nor ever was - required for you to be able to use your machine.

You would obviously be unable to use software that requires internet access - this includes all e-mail software, instant messengers, Web Browsers and filesharing programs - as well as any form of online multiplayer in games or any games that use online-only DRM (examples would be Diablo III, Assassin's Creed and any games that rely on Valve's Steam platform).

Additionally, you would be unable to download software, patches or updates from the net - although there is nothing that prevents you from getting them using removable media (external hard drives, flash memory sticks, optical discs etc) to copy them from a machine that has internet access.

Any software that does not rely on internet access as part of its' basic functionality would remain unaffected.

On the bright side, you would be quite a bit safer from malware.

TL;DR: Same way people have been doing it for over two decades before internet access became commonplace.