Probably the only reasons for a "total newbie" to operate the camera herself would be if: 1) you can't find anyone to do it for you; 2) your access to the subject requires that you work alone; or 3) the very structure of your film depends upon your POV as cameraperson. Other than that, you are not going to be successful, especially in the tougher lighting situations you are talking about. That being said, even if you do have a camera operator on board, you should still get used to whatever camera you do buy -- do shoot practice scenes, and try to shadow an experienced filmmaker first.
Assuming that you are "doing" and "practicing", I heartily agree with others' suggestion to read Michael Rabiger's book. Definitely the best.
While much of the story can be found later on in the edit, you have to have SOME idea of why you are shooting this doc. (I'm sure you do, but you probably don't want to reveal all the details now.) It often helps to write out a short treatment or synopsis of what you envision for the film -- the process of writing sometimes fleshes out the WHY of your film.
Finally, watch a doc every day, and see which styles you appreciate and which ones fit more of the mood and tone you envision for your project. You might decide on the "direct cinema" style of the Maysles Brothers, or the "man behind camera" style of Ross McElwee, or (god forbid) the "pan 'n scan" style of Ken Burns...