The struggle is real when you draw a pretty naked peep and then it's time to add the clothes, why why, but yeah clothes are so pretty and awesome too, so sometimes I need to add clothes u..u .
To answer your question, you draw the body first and then you add the clothes. You see, clothing folds according to the person wearing it, if he/she raises his/her arms you'll have some folding different to, let's say: your character is flexing his/her arms. So you need to visualize what your character is doing to draw fitting folds for their clothes.
You could practice drawing your own clothes, or your friend's. I'm pretty sure you have some pics of yourself wearing some formal-informal clothing. Pics are at party, at school. Try drawing them, see how they fold =] . You'll be drawing pretty awesome clothes in no time.