A few searches should produce many pages online for this subject e.g.
For a straightforward start, you could experiment with a framework such as:
Decent in the main, you may choose to combine stylesheets and worth keeping an eye on the screen break points, times change.
Or improve an existing site, as many non-specialist people have e.g.
Where even a quite simple, older static site can be made to offer reasonable mobile content with a few hours work using media queries. This is not a difficult technique at all. No reason for most site owners not to do this, otherwise many small device users just hit the back button.