Once it has been determined which class you will pick up first, it's time to start planning and organizing.
Get a calendar and block out how many lessons you will need for this grade until the end of your placement.
Begin filling in those blocks with a possible sequence of lesson ideas around a theme/concept. These plans do not need to be fully developed but by searching out and plugging in material at this early stage you will find yourself less overwhelmed as you begin to pick up more classes.
Share these ideas with your cooperating teacher to be sure that they are grade appropriate concepts and materials. Don't be discouraged if you don't have a sense of what is appropriate for a given age level/class. This knowledge comes from experience and practice. If something you select doesn't work for one grade, you may find that it's good for another and can put it in the "maybe later" file!
Continue this process as you pick up more grades.