There are lots of alternatives for bike buying. You can go into a bike shop and find the most expensive one, or the shiniest, or the one with the most radical accessories. You can also go into the bike shop, preferably the larger ones staffed with 17 year olds who could not care less about you or your long-term relationship with either the shop or the bike, and simply tell them how much you want to spend. They will undoubtedly steer you directly to whatever bike has a sales contest running, or that the shop has an over-inventory problem. These approaches will rapidly separate you from your money and get you a bike. If you are in a hurry, I would go with this approach.
On the other hand, if you have a few minutes, here is what I suggest. In a comfortable and pleasant place that is not a bike shop, consider what you want to do on your bike. Just ponder your vision for cycling. Does it involve a future in racing, or does it involve a future in exploring coffee shops? Does it involve riding on dirt and gravel across mountains, or does it involve asphalt trails? How often do you plan to get the wheels off of the ground? How often do you plan to ride it at all - daily, weekly, occasionally in nice weather? It doesn't matter what the answer to any of these questions is. People who like cycling and aren't suffering from self-esteem issues will recognize that there is room for all of these types of cyclists. And the other bit of good news is that there are bicycles specifically manufactured for just the type of riding that you are envisioning.
The bad news is that you have to supply that important part of the process. You have to be able to go into a bike shop that has reasonable and experienced salespeople and then tell them what you want to do with your new bike. The next step isn't too hard, but then you have to "listen". Really listen. And then make a judgment for yourself about whether what you are being told makes sense and really relates to what you want, or, on the other hand it has more to do with the bias of the salesperson and whatever they want to sell. If you sense that you are being "heard", then follow the path to it's logical conclusion. If not, then head to another bike shop and see what someone else has to say. I really don't think it is any harder than that.
Sure, I could type several thousands of words on frame materials, wheel sizes and widths, or styles of road and mountain and lifestyle and cruiser and commuter and whatever bikes, but until you are looking for the fine differences, none of that means a lot. Until then, find a bike shop that is conveniently located, where you find something simpatico with the folks there, and then build a relationship with those folks so they understand what you want, what you need, and make suggestions that fit your needs and budget that are about you and not their inventory.
There, problem solved.
One last suggestion. After you ponder what you want, consider going into Steve's on Cannon and talking to Steve or Larry. Be forewarned that Larry won't have his glasses on, and will squint across the shop at you when you open the door, but he isn't scowling at you, just trying to see you without his glasses. After that, you will find that both of these guys are friendly, know bikes, of all kinds, and can help you figure out what makes sense, whether it is your first, or your twenty-first, bike.