Drawing people, their faces, their hands, their clothes, their hair, everything, is extremely complicated. If you're drawing on a regular sheet of paper, a millimeter off can look strange and you'll have a hard time figuring out why. So many things play off so many other things. For example, the eyes need to be in a relative position to the face, and so do the ears and nose and eyebrows. But it's the almost unseeable differences that make each face unique. Same with the body. The shoulders, the length of the arm, the size of the hand, all need to be a certain proportion to the head for things to look right. You can use guidelines but you rely more on your instincts to get things to not look off.
But you have to start somewhere. And I mean that in two different senses. The first is that you need to start somewhere on a blank canvas, and everything is a play off that foundation. Everybody goes about their own way, so some people sketch out rough shapes to get the proportions down, and some people just go right into the details and somehow manage to get out of it perfectly fine. But think about how important those first strokes are. The first lines dictate where the next hundreds, thousands of lines go next. It's not that you need to think super carefully about the first steps, but know that you need to pay attention to where you're coming from almost as much where you're going so you can make something coherent.
The second thing to get is that for a long time, it's just not going to be that good. You'll make something, and you'll admire it, and then you'll look at others that just blow yours out of the water and you'll get discouraged. That's the most important time to know that these are just your first few strokes. Each new drawing you make is going to be at least a little bit better than the last. It might be too small for you to see. The important thing is how long you manage to fight the discouraging notion that you might not make it there.
Think of each piece you make as a stroke. Each piece you make after that is a stroke that plays off the previous one. The more strokes you make, the smaller those first ones become, and the bigger the picture becomes, which means the more space you have to work your piece into something comprehensible, and finally, in the end, maybe after a thousand or tens of thousands of strokes, something beautiful.