A well-planned lesson is far more likely to lead to effective learning than an unplanned one delivered off the cuff. A well thought out lesson also reduces the probability that serious misbehavior will occur, and makes classroom management much easier. Lesson and unit planning is an art and not a science. It takes much practice to actually be able to visualize the course of a lesson in advance. An experienced teacher can look at a lesson plan and immediately spot words which will be unfamiliar to the students, potential trouble spots where students may get restless, activities that may not be understood by the majority of the students, and so on.
Planning is definitely a learnable skill.
You will be amazed at how much better your lessons go when you have a good plan that results in an educationally effective, fun, and interesting lesson. A great science lesson is a masterpiece and very hard to reproduce. Choose a topic. This can be a single content area of science, i.e. a botany unit, or an interdisciplinary study, e.g. a unit on motion, which cuts across the science disciplines. Specify the grade level the unit is intended for.