[MENTION=302603]bikemig[/MENTION] has great advice, the kind of advice I usually give. However, this can really be tough for a self-described "total newb". If you have a cycling friend who can help you navigate Craigslist, that can help. Without that, CL can be a time-consuming hassle and you might end up with a bike that doesn't fit, etc.
SO...I kinda think that *this* OP will be best served by going to a good local bike shop.
Originally Posted by
bikemig
I'd find a used quality vintage mountain bike for a commute like this. With slick tires, they make darn good commuters and the price is very reasonable. Used mtbs are an under appreciated part of the market and do not command a premium. You can pick up a high end used mtb in the heartland for well under $200; it will cost more in LA but certainly $300 or so should do it. You can show links to the bikes you find on the C&V appraisal forum with your requirements and you will get a lot of feedback....