You may already have good training in how to ride with traffic, but just in case, my first advice is read this: Bicycling Street Smarts - Table of Contents
My next advice is to convince yourself that it’s right: that cars don’t really crash into bicyclists they can see in front of them, and that riding alongside parked cars is more dangerous than riding out in the middle of the lane, even though it might feel safer.
And I’m told that beginners always ride in too high a gear, so they’re pushing too hard. Get a gear so that your feet are moving quickly with a relatively light force, rather than mashing slowly (though I bet the SO has already covered this).
As far as gear goes, a waterproof/breathable jacket and pants (for the pants you can go cheaper and less breathable, cause you don’t sweat as much down there, and the pants will wear out from cycling sooner than the jacket). I also have rain booties that go over my shoes cause I don’t like wet shoes and socks. They’re like $9 from Campmor on line. I think fancy jerseys and what not are kind of silly, but bike shorts can help with chafing if that’s an issue (I don’t wear them). In the fall, you’ll want gloves a lot sooner than you’ll need them for walking (like when you just need a windbreaker), so plan on that, even if you think you’re not ready for riding in arctic weather (though don’t underestimate how much heat your body generates when biking). I like cross-country ski gloves with leather palms and fabric backs. For sizing, remember, none of this needs to be special bike clothing, so don’t limit your options to bike stores (the only bike-specific clothes I have are the booties; the rest is camping/hiking raingear and regular shorts, T-shirts, sweatpants and fleece jacket). And, of course, there’s always the possibility that if you ride hard every day for a few months, you might end up needing smaller sizes.
If your bike hasn’t just been tuned, then go ride around the block, pump up your tires and lube the chain, and ride around the block again noticing the difference. That should be enough to convince you to keep up on that.
That’s the key maintenance to learn. You could learn to adjust your brakes I suppose; that’s probably good to do every few months. Know how to change the tire, too. Though if your commute has public transport, remember you might be able to deal with a flat by locking the bike up, hopping the bus to work, and dealing with the flat later (in fact, that’s my strategy, and today happened to be the once every four years or so I have to put it into action).
After that, assuming nothing breaks, it could use once a year or so having the various bearings relubed and adjusted – but that’s probably something to leave to the shop until you’re ready to invest in specialized tools.
Anybody’s Bike Book used to be the standard book for beginning bike maintenance/repair; if it’s still around I recommend it.