The first draft is the place to explore ideas and different ways of expressing and organizing them. As you write, you may discover new thoughts and connections, and you may decide to change the focus or direction of your paper. The first draft is the place to make those discoveries.
Some people compose their first draft from an outline; others freewrite their thoughts on the subject and organize them after they see what they have to say. Choose whatever method is most comfortable for you. Do not concentrate on making a first draft grammatically or mechanically perfect. Those changes can come later, when you are editing.
When you have finished a first draft, put it aside for a while. Then read it with a fresh eye (or have a peer or colleague read and respond to it), and revise. (See also Composing Process.)