Assessing what to charge is the same in any business - it's about how much you need to earn, how much the market will pay & how much the competition is charging.
Finding out how much the competition are charging also gives you an idea of how much folks will pay (and I guess that TEFL is well-enough paid most anywhere that you can live comfortably) so search for local schools or teachers. I don't know if you're in a town or a city there, so I don't know if it'll be easy to find local competition who'll be directly comparable with what you can offer so if necessary look in the yellow pages in the next town. If you're somewhere remote with fewer potential students then you might have to be prepared to charge a little less to get business moving but if you're in a larger town then don't feel you need to "undercut" the competition.
The first rule of business is not to undercharge!! You can always lower your prices later or run special offers but it is an easy mistake to become known for being cheap. When you do so it becomes difficult to raise your prices as you risk losing your existing customers - start out expensive and people will appreciate your quality! Also remember that you don't earn 8 x your hourly rate each day - aside from the time you need for preparation I'd imagine that TEFL is draining and mentally quite hard work, so full 8 hour days are probably to be avoided; I'd guess that if you charge enough that you can afford to work only 4 hours a day then you'll be much more relaxed and consequently a better teacher.
There must be work coaching 16 - 18 year-old students in any town there's a high school, so put up adverts in newsagents near the school - hopefully parents will see them & contact you.