As a teacher, planning and preparation is vital to ensure the smooth running of a classroom. It is very hard to teach without having planned your lesson in detail and getting prepared any materials you will need.
Most teachers have a basic plan of what they will need to be covering within a term or half term and then on a weekly basis, split this down in to specifically what to teach within that week. You can often refer to previous lesson plans from earlier years to base your planning on but as educational targets and views change, you may need to tweak this each time you use it. There are also some great places to find resources to help you with your lesson plans and advice from other teachers on how to tech a specific subject in an informative and fun way.
Always ensure you have time put aside each week to go over the next weeks plan and double check it.