start up, startup

Don't use start up as a verb. Use start instead.

Note Use open, not start, to describe selecting a program or app from the Start menu to open it.

Don't use on startup or similar noun phrases in content for a general audience. It's OK to use on startup in content for a technical audience.

It's OK to use startup as an adjective in phrases such as startup screen.

Examples
To open the app, select the icon.
When the app opens, a startup screen appears.
Open OneNote and start taking notes.

See also Start button, Start menu