Cycling changed my life dramatically, and for the better. A lot better.
First, you need a good bike. You really don't want racy unless you are about to race. That means you want relaxed geometry. You don't have to spend a fortune, but I wouldn't go much under a grand.You can get really good bikes between 1-1.5K, cheaper they are, the more compromises they have to make.
But the very first thing you need to do is find the best bike shop in your area. It's usually not hard to tell, they'll be interested in you, not just your wallet.
Now for the tricky stuff. It's best to have someone teach you how to deal with traffic as an adult. You can read up on it, but bike shops usually have group rides, and you can see what people do, and ask question.
The saddles, even on expensive bikes, are usually crap. But don't get one right off, it takes a couple weeks to toughen up your butt. I'd skip this the first year, but if you hate flats like I do, you get kinda fanatic about tires.
But there's a catch, some quality tires ride like a truck, and you don't want that either. There are a lot of variables, how abrasive the roads are where you live, do plants give off thorns that puncture tires (or are the roads dirty, which means they'll have things that can put a hole in your tire).
If you get a flat (make sure you have a flat kit) and decide you hate flats, I'll provide a quick alternative: Schwalbe. For my 2 cents, there are only 3 tires sizes in 700c. 28/30/32.
Which is not to say anything bad about 26 inch tires. Especially if you are short, the smaller wheel keeps the geometry normal.
I always wear a helmet.
My technique for riding is when I get in traffic, I act kinda like a cop. If I am going to merge, I make eye contact with the guy behind me, signal, then turn. I am often assertive, directing traffic if I need to, but never, ever aggressive. That's going to sound crazy to you right now, but once you figure it out, it becomes second nature. Most people just want to know what to expect.
About bike fit, most people buy their first as an adult a size too small. 2 weeks later, they wonder why things aren't going the way they should.
"There is, however, a position that allows good performance while minimizing risk of lower back injury. I like a stem height and length that puts your back about 50 degrees from horizontal, while your arms and legs bend slightly at the elbows, as shown in figure 2 up there. To achieve this, you'll probably have to raise your bars, and assuming you want to keep the same bar style (as opposed to riding with stingray bars or something), that usually means getting another stem, one with a taller quill or a steep rise to it. If you hit the sweet spot, a photo of you from the side will reveal a nice pyramid composed of top tube, torso and arms."Bob Gordan
As you know 45 degrees is halfway between flat and upright. When he says 50, he means almost halfway. But if you are in good shape, you can go to 40 or 45 degrees. But if you do, consider a professional fitting.