You need to let people know that you exist. There are several ways to accomplish this.
1. Place an ad in a local newspaper or an advertisement in the school's student newspaper.
2. Have business cards made and post these on school bulletin boards.
3. Talk with the school's headmaster or principal and let them know you are available for tutoring.
4. Word of mouth advertising is the best. One student tells another and so forth.
5. Internet chat sites are excellent. YouTube, Twitter, etc., will reach lots of people.
6. Post a short video of yourself on YouTube explaining what you do.
7. Start a blog or website. Students can ask you questions and, for a small fee, you will answer their questions. The content of the blog or website is up to you, but don't stray from your area of expertise. If you tutor in physical sciences, don't start discussing the latest fashions in Bollywood.
These are a few ideas. But direct contact is always the best. In this respect you are like a salesman going door to door. Only you are selling your knowledge as a tutor.