I am not an engineer but I want to make a small radio circuit. Which wire would be good for its coil.
You should probably use magnet wire. That is copper wire that is insulated with a thin but tough varnish coating. The coating must be scraped or sanded off wherever you need to make a connection. That may be particularly tedious for a coil that needs to have provisions for a contact that slides along the coil to select a connection point on any turn of the coil. It may be possible to make such a coil with bare copper wire, but that requires very careful winding to prevent any turn from touching the ones next to it. That would be a very tedious alternative.
Note that magnet wire is widely available as "magnet wire." It may be described as "enameled wire" also, but that older term seems to be less common.