If you never draw but always wanted to, the easiest way you can make a nice drawing from a photo is to use a technique from the time of Michelangelo. You grid the photo into accurate squares and copy the lines accordingly. That will undoubtedly give you a very reasonable result of likeness. The same technique applies to shades. If you make a copy of the photo in black and white it will show you the exact depth of shadows. So you can copy in whatever color you need.
If you already started to draw and want to make it a bit more than a copy of the photo, then you have to study the person closely. It's because the artist is a bit of a psychologist. That's the main reason I am not fond of art classes with live models. Yes, you have someone to draw who sits still. But he or she just oozes boredom. Imagine sitting stiff for a few hours starting at the same spot unable to move!
That's why I prefer to sketch on the streets. Yes, some models escape fast. Sometimes I see someone I really like, start to sketch and just hope that they will hold a position long enough for me to finish. Because that's where magic happens.
You see a strange twist of hand that shows deep mannerism of someone who seems to be a sweet nice girl. You see a guy with a beard and manly shoulders, but the beard hides a childlike mouth of an inexperienced youngster. You see a lady with impeccable hairstyle and rings on the fingers who is deep down really just a little girl because you noticed the enjoyment she had sipping her cocktail. Yes, she even twisted her foot like a little girl would do when she got candy.
Each face, gesture, position tells the story. You have to train both your hands and an eye to see it. It is difficult. If you feel it's too much, start with a tree. Yes, a tree. It doesn't go anywhere, so it's easier. That will take some of that fright away, because sketching live is a bit scary. It takes some practice to stop worrying about onlookers and possible comments.