Observation is the first step in successfully painting people. Watching how people sit, stand, walk, and interact, what clothes they're wearing and how these hang on their bodies, how they carry bags, where shadows are created... The advantage of watching people in real life over working from photos is that you're creating visual memories you can use later to paint figures. Instead of hunting through photos, you'll hunt through your memory banks and use the info you've collected as the basis for adding people to a painting where you want them.
The disadvantage of painting people from life rather than photos is that people move about all the time. At first, it seems they're never going to be still long enough to capture the whole figure. With time, you'll get faster at it, but initially the thing to do is to find people in a setting where they're going to be relatively still. Such as in a queue somewhere, waiting for public transport, or sitting in a coffee shop.
Keep things simple when you first start, working with line only, like this.