I'm a long time bike commuter guy (20+ years) and bike junkie, I have to disagree with some of the advice here:
-Do not buy more bike or a more aggressive fit than you need for where you are at right now. A bike is a tool for a job, and that tool has to match your comfort level and abilities. If you decide you love this, you can always trade-in/sell/upgrade. Buy something that you are confident riding out the door today.
-You do not need disc brakes right now. Yes they're great but you're probably not going to notice the difference for getting started. They are more expensive, require more complex maintenance, and if something happens to them on the side of the road, your average bear doesn't know how to help you (or they might try and make a bigger mess).
-Similarly, you don't need tubeless tires. Don't even get into what that is and all the different tire types. Look at the hybrid bikes at your local shop and ask them how you change the tire should you get a flat. That's all you need to know.
-Do not buy off Craigslist. Bike can have dangerous issues like cracked bottom brackets, etc, that takes an experienced eye to look over before purchase.
I'm +1 on both Recycled Cycles and Montlake Bicycles being great for beginners. Montlake Bicycles is typically less busy/calmer than Recycled if that helps you feel less overwhelmed.
What you're going to want most is a dry ass. Fenders and those plastic rain pants. You will want at least one waterproof bag for your essentials--laptop, wallet, phone, work clothes--because when it decides to pour, it pours. REI Outlet online is a good place to hunt for both these on discount to get started.