Arpad asks, "I already have a visitor visa to the United States and I want to go there next fall. Then I want to join a university and apply for change of status to student visa. Is it possible?"
Yes, USCIS does allow an alien to change status from B1 to F1 (or M1 in case of vocational studies) but you must do the paperwork and be approved before starting classes. If you join classes before your approval, you maybe accused of violating the terms of your status, your application rejected, and you might be put in deportation proceedings. Also, because you entered the country with tourism intent and did not disclose that you actually planned to study, the agents will examine your application more carefully and reject it if they suspect that you have no intention of leaving the country on graduation. However, those people who came to America just for sightseeing and after spending a few months realized that they would love to actually study here may have a better chance of getting approved without a hitch. A rejection, however, does not mean that you cannot go return your country and apply again.