English courses for foreign people are called ESL (English as a second language). Courses can be held for day classes or night classes. Depending on previous experience and price, the classes may be for up to 8 hours 5 days a week or as little as an hour once a week. Many schools offer the opportunity to take a TOEFL tests to prove your competence at the end of the course.
If you already speak English fairly well, ESL courses are also offered in higher education institutions from community colleges to universities. Community colleges are much less expensive and will give very similar training. There are also classes offered at community centers that can be extremely inexpensive, but may be crowded and do enable you to earn a student visa.
Some well-known language centers:
Converse International School of Language
605 Market Street, Suite 1000
San Francisco, California 94105
Learn 4 Good
1462 Pine Street
San Francisco, CA 94017
Berlitz Language School
180 Montgomery St # 1580
San Francisco, CA 94104
For a list of language centers in San Francisco: sanfrancisco.citysearch.com/yellowpages/directory.