1. Buy a bike that's the right size and fits well. You could go with upright, that's a little easier to get fit on. Or entry level road, like a Specialized Sectuer. There are definitely cheaper options out there as well. Just don't get something that isn't the right size.
2. Did I mention get it fit right? Lol, #1 mistake I saw in college was people riding with the seat to low. I don't mean an overly complex $250 fit with video, just something that puts your seat and handlebars in the right position.
3. Make sure your bike has flat resistant tires. Most bikes comes with them now, but I wouldn't ride without them for anything. You probably also need tools to fix a flat on the road...it somewhat depends on your route.
4. Lights. If you're riding at or around dark, you need a front light and a rear blinking light. I use 2 rear lights because I've had one go out, and you don't know because it's behind you so you're riding with no lights. People have recently been talking about how cars seem to see them better having their front light blinking in daylight as well.
5. If you want to ride in inclement weather, that adds a lot more complexity.