You become a Southern Baptist by uniting with a Southern Baptist church, one in friendly cooperation with the general Southern Baptist enterprise of reaching the world for Christ. Typically, church membership is a matter of receiving Jesus as your Savior and Lord and experiencing believer's baptism by immersion. Southern Baptists have prepared a statement of generally held convictions called The Baptist Faith and Message. It serves as a guide to understanding who they are.
Below is a short summary of key doctrines of the Baptist Faith and Message. The complete Baptist Faith and Message, with scripture references, can be found at this link https://bfm.sbc.net/bfm2000/