Determine three important factors:
Your goal for your studies in microbiology: Will you take only one or two courses, or do you want a degree in microbiology?
Do you have a science background that allows you to take advanced courses, or do you need a beginner's level course?
Are you willing to pay for credit-bearing courses, or do you want to study independently and receive no credit?
Search for a microbiology course or e-book online that fits your needs based on your answers to the previous questions.
The Open University, offering only online courses to students anywhere in the world, has a food microbiology course.
Colorado Community Colleges Online also offers a microbiology course online for credit, and it is geared toward people entering the health sciences field.
To study on your own at an advanced level, use the free and in-depth e-book offered by the University of South Carolina School of Medicine website, which is linked below. Also use Massachusetts Institute of Technology's (MIT's) free, non-credit OpenCourseWare.
Keep to the schedule fixed by the online course syllabus, or create your own schedule and weekly study goals to complete your studies in a timely manner. Studying online requires strong self-motivation. If you paid for the course, use that as your motivation to complete the course.