Poke around Craigslist and FB Marketplace for frame sizes around 48cm, 50cm, or 52cm for around $400-500. Once you find one you like, go test ride it.

I typically recommend single speeds for starting out on since they're low-maintenance and you can still get a good workout by just going places (working your way up hills and whatnot). If you opt for gears, you'll likely find bikes that come with either Shimano or SRAM groupsets.

I'd also budget an extra $100 or so for a helmet that you think looks cool (note that some of the more slick road helmets can get kinda pricy, but it sure beats brain damage imo), and eventually a headlight and taillight once you have more experience