There are obviously may issues that go into choosing your first motorcycle such as style, make, model, size, cost, etc... But there are a few general issues that should be kept in mind when choosing your bike.
First things first. The motorcycle you buy has to fit the experience you have and your body size, leg length etc. If you are just beginning you need to ride a motorcycle fit for beginners like a small 125cc motorcycle. You need to ride it a large parking lot that is vacant so that you get used to all the gears, turning, stopping quick, etc. Before you start riding in the parking lot, you need to buy the right riding gear. A DOT helmet is a must for safety of the brain. A full face helmet is the safest at first because it totally protects the face. Gloves, and decent boots are a must for riding and should not be forgotten.
The next thing you need to do is take a motorcycle safety class and while you are at it and pass the motorcycle safety class take the advanced class. Not only will these classes teach even the most advanced riders some things, if you are a new rider the class will generally work as the DMV riding portion of your motorcycle licensing.
You are now at the advanced beginners stage and you have ridden your 125 and are comfortable with it, you can stop it, control it and know the laws for riding motorcycles. Most importantly you feel safe in traffic and you have ridden enough. I would spend at least six months riding a 125 to 250 sized bike on the streets so you feel safe.
What should be your next motorcycle? That really depends on what you want to use a motorcycle for. For instance I love to travel the U.S. on a motorcycle so I have a bagger which can take luggage etc. Or you may want to use your motorcycle for just getting to work and back. There are many bikes that fit that bill made by different manufacturers. Maybe you want to ride the canyons and that would take a sports bike and there are many of them. Maybe you want to ride in the dirt and compete on dirt tracks and there are many of those bikes on the market.
Whatever bike you purchase it must fit. You must be able to sit on it comfortably with both feet completely on the ground. You must also be able to reach with comfort all the controls on the motorcycle, i.e. the handlebars, the switches, the brakes etc. You must also be able to see clearly through the windshield if there is one and be able to feel comfortable at speed on the motorcycle. All motorcycles must be test driven over a long period so you have a comfort level with them