If you are planning to take classes in the United States, the first type of visa you are likely to look into is a student visa. But you will quickly discover that the F-1 visa is for academic students entering a full-time program, usually at a college or university, which must have received U.S. government certification through the Student Exchange Visitor Program (SEVP). The lesser-known M-1 visa, for vocational students, also requires full-time study at an approved program.
But what if your plans are for more casual or fun study? For example, a few weeks of cooking, yoga or art classes? Or perhaps you just want to take a short or part-time academic course and spend the rest of your time traveling in the United States.
For the purposes described above, a B-2 tourist visa might be sufficient. (See 22 C.F.R. § 41.31.) If so, it is also preferable to applying for an F-1 or M-1 student visa. The application process will be shorter and simpler.
The main thing to consider in deciding which visa you qualify for is whether or not your planned course of study lasts 18 hours or more a week. If so, you will need a student visa; if not, you might qualify for a tourist visa. Talk to the school or program you will be attending, which should have information about the past experience of foreign students attending their school. If that doesn't get you the answers you need, talk to your local U.S. consulate.
If you decide the most likely strategy is for you to enter the U.S. as a tourist, you will need to fit the normal eligibility criteria for a tourist visa, described in A B-2 Visa for Visiting the U.S. as a Tourist: Do You Qualify?.
When you apply, you will have to show that your trip is for the purpose of pleasure and that you do not plan to stay in the United States permanently. The maximum stay you will be allowed is usually six months, but if your course lasts longer, you might be allowed up to one year. You can ask for permission to stay longer than six months when you are being admitted at the port of entry (a U.S. airport or border facility, most likely).
When you get your tourist visa, the consulate should put a special notation in it regarding your study plans: "Study incidental to visit: I-20 not required." Examine your visa when you get it, and request such a notation if it's not there. This notation will avoid confusion at the border and in the United States, by showing that you aren't misusing a visa meant primarily for tourists. (The "I-20" is a form that academic and vocational students must receive from their school before applying for their visa.)
If the consulate doesn't put this notation in your visa, this shouldn't stop you from entering the United States. When you get to the airport, border, or other port of entry, however, be very sure to state that the main purpose of your trip is to be a tourist. You can mention the classes later, but if you start out by saying, "I'm here to study," the border patrol officials will quiz you on why you didn't get a student visa.