You can teach, in which you have best knowledge and about which you are very confident.
Like,
Competitive Exams ( Bank Exams, Railway, UPSC, NET, SSC, State level Govt. Exams etc.)
Entrance Exams ( JEE, CPT, CAT, CMAT, AIEEE, CPT etc )
Computer Languages ( Java, C language, C++, C#,PHP,Python, Ruby, Android etc )
Languages (English, IELTS, TOEFL etc. )
Skill course ( Yoga, Excel, Interview, body language, Personality Development, Public speaking etc )
Any subject of school or college ( Maths, Social science,Economy, Account etc )
Share your knowledge to audience with attraction. Use your imagination and skill to bring your subject in a way that students will always remember.
As the author of the course, it is your responsibility to ensure that the material presents a technically as well as grammatically correct.