It's one thing to be a volunteer tutor. It's another thing to lose money tutoring students. If you're not charging anything for tutoring, you shouldn't have to pay to advertise your services.
Most student newspapers charge for ads.
Look for a tutoring board in your Student Union Building. If you ask the people in charge of these listings, they will usually direct you to a billboard where tutors can advertise their services. Sometimes you need to write your ad on an approved advertisement card, provided by those in charge. I am offering free assistance with introductory Math and Computer Science courses. Whether you just need help with one assignment or need to review the whole course, give me a call.