How do you become an anthropologist?

If you want to become an anthropologist, there is only one thing you can do. Go to school. Call up your local university or college and get more information about their degree programs.

You can also go to your local library or bookstore and pick up books on anthropology. Start out with books that give you a general idea of the field. As you learn more, narrow your research to things that interest you.

If you want to become involved with anthropology, you have several other routes you can take.

Read, read and read some more. There is a wealth of books on anthropology, from overview books describing what anthropology is to books on fields as specific as the mating rituals of some far off islanders. You can glean an understanding of the field pretty quickly.
Subscribe to anthropology journals. The American Anthropology Association has listings of many, if not all, of the journals currently being published. I recommend "Cultural Anthropology" for anyone who is interested in the cultural anthropology. There are other subfields of anthropology that may interest you.
You can always check your local universities for any volunteer work related to anthropology. You may find opportunities to conduct surveys, participate in a dig, or help out with some research. Don't get your heart set on becoming Indian Jones though. More than likely, the most exciting thing you'll be doing is sifting dirt.
Join a local club for anthropologists. Again, these clubs are generally run by students from the local university anthropology department. It seems like some of these clubs allow anyone to join. You can get information on local events and national meetings.