Genuine TEFL diploma courses (e.g. Cambridge Delta and Trinity Dip TESOL) are only for people with considerable experience and (usually) an initial teaching qualification. Calling anything else a "diploma" is usually a sign that a course should be avoided.
Your three options are therefore to take a TEFL certificate such as a CELTA, an online TEFL course, or both. For most people, the best option is to take a TEFL certificate with at least 100 hours of face to face instruction and at least 6 hours of observed teaching practice straight off. The Cambridge CELTA is the most well-known of these. If you can't do that, e.g. you can't quit your job for at least another six months, then it can't hurt to take a purely online course to help you prepare for a CELTA. I would not recommend job hunting with just an online qualification though.