How do I make characters show as bold on my iPad on screen keyboard?

You could go to Settings > Accessibility > Display & Text Size and turn on Bold Text but this will make also all the displayed text bold.

Text for formatting may be accessed in different ways - as determined by the App that your are using.

By example, for the native Mail App - the text formatting tools are accessed from the aA button located within the on-screen keyboard...

Other Apps may provide different (but similar) ways to access and control text formatting options.