Every TEFL lesson needs a plan. The level of detail it contains, and whether it is mainly in your head or mainly on paper, will vary depending on your training and experience, the type of class (one-to-one classes often have a much more fluid plan, for example) and the time that you have available to plan. The main reason to have a plan for a TEFL lesson is to know, firstly, the aim of your lesson and, secondly, what you're going to do during the lesson in order to achieve that aim. If you don't know what you want your students to be able to do by the end of the lesson, you risk them going away feeling that they haven't achieved anything.