As far as I know, you can't get "fired" for anything beyond violating the TOS :). There is no specific time commitment -- I've been here for years, and sometimes gone as long as 6 months without doing anything -- and it's open for you to explore your areas of interest. Remember that you're cutting your knowledge down into basically 2-page articles (approximately 1000-2000 words), so you might be surprised how many topics you actually ARE an expert in now. There's also absolutely nothing wrong with picking an area of interest, then writing your hub based on a mix of experience and research from reliable sources. Then you get the personal bonus of learning about something you enjoy while potentially getting paid for the effort. You'll also find that a lot of hub types lend themselves to opinion more than fact, or allow you to put your own interpretation on events.
As for coherence -- I find that writing subheadings first helps me stay organized :). It's also nice that it's really easy to edit here. If you publish something and then decide it's not quite what you like, or learn more on the topic later, just go back and update your hub. Welcome, and I look forward to seeing what you write :D