Professional photographers have enough experience creating compelling images that they don't have to think much about how to make a photo look great. It takes a lot of practice before it all becomes automatic, but there are a few things you can keep in mind to make your photos look more professional starting today.
Style develops naturally as you train your eye and hone your vision. Each of us sees the world in a unique way. As you gain experience, you get better and better at visualizing your photograph to meet that preconceived ideal. As you pick up more skill and technique, you'll see that there are an infinite number of ways to photograph any subject.
The next time you pull out your camera, consciously try to photograph your subject in as many ways as possible. Try different settings and angles. Try it with and without flash, up close, and far away. Do this enough, you'll start to develop your own set of stylistic preferences until each of your photographs takes on part of your personality.
Some documentary style photographers have the mistaken idea that a scene can and should be recorded objectively. That simply isn't the case. Even if you try to avoid developing a style, that itself will inform your style. You can't help but bring your own perspective to the subjects that you shoot, so you may as well do it consciously.
Perhaps related to style are the subjects you choose to photograph. Whatever subjects you choose, be sure that you are passionate about what you shoot. Photojournalists may object at this notion, because they might be sent to shoot a basketball game one day, and a crime scene the next. In the case of photojournalists, they should be passionate about finding and telling stories of the subject they cover. If you're an enthusiast, simply shoot whatever interests you. If that's music, take your camera to concerts. If it's airplanes, you should be at the airport every day. Shoot what you love.