There's the sdk manual that also has a link to some plugin templates you can download
https://www.scirra.com/manual/15/sdk
and you'll probably want a javascript tutorial:
https://www.tutorialspoint.com/javascript/
I also seem to recall some guides you can find here:
https://www.scirra.com/tutorials
Beyond all that the time required to to make a plugin depends on the plugin. In general it's easier to just do what you want to do with events or an existing plugin/behavior, and there are a lot of them.