1) you find a school that accepts you into a program.
2) you get an I-20 form from the school
3) you fill out a form I-539 and include all the supporting documents in it and send it to the USCIS office. You will find this form on uscis.gov website.
or
4) you leave the States and come back on F-1 visa, since change of status may take 3-4 months.
If I were you I'd hurry, since a lot of bigger better schools have a deadline in January for applying for the NEXT fall semester. Smaller community colleges may be more flexible and you may even start school in F-1 status in January (if you leave the states and come back on F-1 visa).