When you decide to begin working with people, animals, or anything that has a mind of its own, be prepared to engage in a lot of trial and error. This trial and error period will most likely last forever, due to the volatile nature of living beings. Really though, what you want to do is build a certain comfort level within yourself in order to get involved with how you think the people in your pictures should appear. This type of thing takes a commanding attitude, lots of confidence and tons of practice.
I suggest you begin with approaching random people on the street while you’re out taking photos. Ask them if you may take a few photos of them doing different things. See what they say. You’d be surprised how many people would agree. Explain to them that you’re practicing your photography and that you’d like them to pose in certain ways for you while doing certain things. This will undoubtedly build your confidence on so many levels. It’ll also teach you how to deal with various personalities. One of the most important aspects of working with humans as subjects is your ability to communicate and to express to them how you’d like the ultimate image to appear. See if you can get your subject to follow instructions and to work with you effectively. This entire area of photography is more of an art form than anything else. Shoot lots and shoot often. The more practice you’re able to get with random strangers, the better you’ll be with paying customers. When someone is paying for your services, they expect professionalism, so that’s what you want to show them.
Most important is your goal for your shots. How do you want them to look? What message are you trying to send? How can you communicate that message to the people in the shot? All of these things take practice, so head out there as soon as possible to get started.