Becoming a psychologist is a long and arduous path. A psychologist is an expert on human behavior, thinking, and mental processes. In most states, the title "psychologist" is reserved for someone who has earned a PhD in psychology and has attained the necessary certification. Be prepared for a long period of schooling if you expect to become a psychologist.
In high school be sure to get the best grades that you possibly can. Take a wide array of classes that interest you and do well in them. If your school offers any electives in psychology be sure to take those as well. The most important part of high school, other than doing well in your classes, is getting involved with leadership and volunteer organizations. Now is the time to explore any interests that you have that pertain to psychology. Right now your primary goal is to get into the college that has the psychology program that you want to attend.
Most psychologists get a bachelor's degree in psychology with a minor in any other field that interests them. However, many psychology graduate programs do not require an undergraduate degree in psychology. If you decide to enter the psychology field after having already obtained a non-relevant bachelor's degree, don't worry too much. If, however, you're entering your undergrad studies with the expectation of being a psychologist some day, you will gain valuable experience by getting a psychology bachelor's degree right off the bat.
Undergrad is a good time to begin networking and building relationships with professors and classmates that are also studying psychology. If you can get involved with psychology research programs and experiments as a volunteer (or even participant) you will be getting yourself valuable experience. Many psychology professors need student volunteers to help set up and run their experiments. Even more importantly, you will need the support of these professors once you being applying to graduate schools so now is the time to build those relationships.
You will want to research the institution at which you pursue your post graduate degree very carefully. At this point you will probably want to begin narrowing down your interests into a particular type of psychology. If you're interested in child psychology, educational, positive, cognitive, criminal or any other type of specific psychological field, now is the time to decide. You will want to make sure that the institutions you are applying to actually have programs in your chosen field or have faculty that are interested in that topic. Don't make the mistake of applying to a psychology master's or PhD program only to find out that there are no faculty there that share your interests. Do your research ahead of time to make sure that doesn't happen.
Once you're accepted into a program you will have to work hard to make sure you do well. If you're in a master's program try to get involved with research projects even if they aren't required. If you're in a PhD program you'll have to do your own original research anyway, so get started early. Jot down any possible research questions as you think of them so that you have a long list of possible projects to undertake.
Most PhD programs take 5-7 years to complete, although once you finish your mandatory classes its mostly up to you and the nature of your research to determine how much longer it takes for you to receive your degree. Depending on your program you will have to write a dissertation, defend it, and possibly put together a portfolio of other aspects of your work to finally be given your PhD.