There are four main things to consider when buying a bike:
1. How and for what purpose are you going to use the bike?
Are you planning on using it for day-to-day commuting? Do you want to get away for the weekend and tour from state to state? Are you passionate and do you idolize Valentino Rossi or Marc Marquez and want to drop a knee on a racetrack or zoom around your local canyon road? Did you get the travel bug after watching Long Way Round and now want to ride around the globe? These questions should serve as your ultimate barometer because there are many bikes out there designed to suit a particular riding style and function.
2. How much bike can you honestly handle?
While you're still learning to operate a motorcycle well, it's important to not be hindered by a bike that is too big, too heavy, or too powerful. Knowing what you can manage not just physically, but mentally, is very important. Ensuring you can place both feet firmly on the ground and reach the controls is a good start, but also think about how good your hand-eye coordination is. If you buy a bike that is powerful and fast, do you have the discipline to refrain from doing stupid, stupid things with that power?
3. How much money do you really want to spend on your first bike?
"Dropping" a motorcycle when you are new to riding is a reality. Damage to a bike means having to fix it, which costs money, and smaller, less expensive bikes are easier and cheaper to fix compared to their larger counterparts. Also, larger engines typically mean higher insurance payments for the first three years you have your license.
4. Do you love it?
The last, but in many ways, most important factor to consider, is this: When you look at the bike you have chosen, does it speak to you? Does it motivate you to get out and ride and experience the world around you? When you walk up to the bike, or walk away from it after a ride, do you have a hard time looking away? Do you think to yourself, "Damn, that's one beautiful and amazing motorcycle, and whoever gets to ride it is one lucky SOB?"
If so, you've chosen well, my friend.