Good luck with that.
Writing tends to be a lonely, self-taught profession.
You'll find lots of mentors on this site just by reading the posts and putting up samples for critique. That's way MORE than I ever had starting out. But ultimately the best way for you to learn is just plunge in and do it.
You are not the first to propose this sort of thing; I've heard variations of it over the years and so far as I know those would-be writers are still waiting for someone to swoop in and help them.
Unless a previously published writer with an agent is independently wealthy and bursting with saint-like generosity, he or she is just going to be too busy with their own work to partner up with a learner.
Don't wait on them. Just get started. In this vocation you learn by DOING.
An artist may take an art course to learn to mix paints, but he doesn't expect his teacher to fill in the canvas, then place it in the gallery for him.
Get a copy of Strunk and White's Elements of Style, read a LOT to see how other writers did it, and start writing for two hours a day, every day, for two weeks and see what happens.
(Two hours a day is a very LIGHT load for a pro writer--something I got from Joe Bob Briggs.)
Again--good luck.