There is no silver bullet to making a professional picture. It often takes years of practice; however, as a newer photographer you'll find that there are some things that will dramatically improve your photography without much effort.
But after you apply the tips on this page and get to the point of being a competent photographer, you'll find that your progress will be slower and you'll have to work harder to make your photo just 1 or 2% better.
But here are the simplest things you can do to quickly see an improvement in your photos.
Capture Interesting Lighting
Lighting is number one on this list for a reason. It's easily the most overlooked aspect of photography for beginning photographers, but professional photographers obsess about it. Lighting is the most important thing to making a photo look professional.
Now what makes lighting look interesting? First of all, it's different from what we normally see. Second, it highlights the subject and presents it in a good way.
There are many ways to make a portrait photo have more interesting lighting. You could simply place the person in the shade in order to put even lighting on their face, or you could turn their back to the sun so that hard shadows don't rake across the face.