For most online or in-person TESOL courses, the only requirement to sign up is to have an English language level sufficient to succeed in the course. For non-native English speakers, the recommended level for certification usually ranges from a B1 to C2 CEFR level (CEFR is the Common European Framework of Reference, an internationally used standard for describing language ability level).
Each TESOL/TEFL provider will have its own guidelines regarding the required English level, which can vary somewhat depending on the academic rigor of the course. For example, Bridge has different English language requirements for our online TEFL courses (including our standard-level, self-study online courses and our university-affiliated IDELTOnline™) than for our classroom-based TEFL/TESOL courses. These guidelines are outlined below and can be found in our TEFL FAQs.
Some TEFL/TESOL programs are more advanced, or even university level (offering credit), and may, therefore, have additional requirements beyond language level, such as a minimum education level (high school or university diploma).