Figure out what your career goals actually are. Odds are, you already have an idea but you might not have sat down and really thought about it yet. Write down these goals for your own reference. For now, don't worry about form or sentence structure. Just get your ideas down on paper.
Relate your postdoctoral career goals to the program you're attending. Edit what you wrote down so that it matches up with what you will be getting from the program. For example, if you are entering a doctoral program for clinical psychology that focuses on research, then your postdoctoral career goals in your statement of purpose should focus on research, as well, even if you plan to pursue other things in your career.
Format what you have written in about two paragraphs, and include the information needed for your statement of purpose. The first paragraph should explain what your career goals are and why you chose these as your goals. The second paragraph should include details about your past academic achievements and experience that backup your career goals. For example, someone interested in clinical psychology research has probably completed fieldwork in research psychology or clinical psychology or participated in several research studies.
Proofread your two paragraphs. Doctoral programs are highly competitive and your career goals section of your statement of purpose are going to be highly scrutinized. You need to make sure that you have used the best possible wording and clear sentence structure and that you have included all the information necessary to clearly articulate your postdoctoral career goals.