So far, I've only written one short story but I wish to write a novella. I'm worried, however, about the enormous work of marketing and contacting agents and publishers. I just want to focus on my writing and let someone else handle the that type of work. Can I hire a manager in my case? and how do managers get paid?
You certainly can --someone will always be willing to take your money --but there are multiple reasons this is a bad idea:
1) You're putting the cart before the horse: You're managing a writing career before you've produced much at all in the way of writing. Until you've done some more writing, gotten some feedback, and attempted some sales on your own, you won't have much idea what kind of writing suits you, how good you are, how your readers will respond or whether or not you'll want to stick with it.
2) You have nothing to manage: See above --you wouldn't be giving your manager anything to work with.
3) Most reputable people in the publishing world work on a percentage of sales not on a for-hire basis. And there's no way the percentage on what you're talking about would make it worth anyone's time.
If you're looking at this as something that will pay for itself, it won't, not on the back of one short story and a novella. But don't be disheartened. Selling is a part of the writer's journey, and it isn't so very horrible. Once you have an established career, then it might be time to revisited the idea of a manager.
Probably Thinking of a Literary Agent
The role you are describing is really that of a Literary Agent. Part of the question is, "Would a Literary Agent take on an unknown and begin representing her/him?"
Great Resource For Leaning About Literary Agents Expectations
That is where the Writer's Market (Writer's Digest Books) will come in very handy. It lists agents who are willing to work with new(er) authors. There's also this one that focuses specifically on literary agents (Writer's Market Guide to Literary Agents 2019)
But you will need a body of work to use as samples to entice any agent since the agent's pay will be dependent upon making sales of your work : the agent will have an understanding if 1) your writing is good 2) your writing is marketable.
Those are two distinct things and the agent will need to be convinced of both to take you on.