It's important when planning that you know what your objective is. An objective is similiar to a goal. By the end of your lesson, what do you want your students to be able to do? During the lesson, what are the students doing?
Having a clearly stated objective helps students focus their learning. In my classroom the objective is stated and written. Every morning all of my objectives for the day are written on my side board. I make sure I draw the students attention to it before every lesson.
Being able to write a good objective takes practice. Many times I find myself writing what assignment the students are going to do... Here's an example of a "bad" objective
Objective: Be able to understand animal adaptations.
How do you know if they understand it? What will they do to show you they understand it?
Here is the same objective written in a "good" way
Objective: Draw a picture of an animal and include any adaptations it would need to survive in its environment. Write a one paragraph essay describing these adaptations and why they are needed.
The most important thing to remember is to use verbs and make sure your objective is observable.