If I am remembering correctly from her other post, PolyPaulie is 20 years old, openly dating more than one person, and has just divested herself of a transphobic boyfriend who was not thrilled with her being poly. (Yay!)
Therefore, it sounds like a lot of people in her life already know she is poly and that she's having no trouble explaining it to people she dates. So I will just talk about being open with family & co-workers.
Co-workers: depends what kind of job and what kind of social environment. In my ideal world, one's co-workers would know nothing at all about one's personal life. In reality, though, people chat & ask questions and there is no way to avoid that friendly office banter. With a full time job, you might spend the MAJORITY of your time with co-workers, more than with other people in your life! In my own new office job, I have managed to avoid answering any questions about my dating life so far, but my co-workers already wonder why I'm so weird and secretive. This is a job I'm planning to stay in for years, so I haven't figured out what to do about that yet.
Family: I alternate between wanting to be more open with them and wanting them to know nothing at all about my dating life. I am not yet sure what the balance is.
So I don't really have any advice for you. Keep reading on this site, especially in the Life Stories & Blogs section. Lots of good stuff there.
PS: I congratulate you for being independent and out of your parents' house at such a young age!