It really is very difficult to say much you should be charging. It depends on where you live, who your target students are and your professional experience.
I suggest you look at what your competitors charge. Take a look at their websites, check out ads in local newspapers, etc. You could also call to ask prices. Obviously, you'll need to think of a reason why you are phoning to enquire about English lessons; maybe you're calling on behalf of a friend. You could also find out what teachers of other languages charge. I suggest you have different rates for group and individual classes. This will depend on the market average.
Take a close look at your costs, including travel time, materials, advertising and any taxes you'll have to pay. This will give you an idea of the minimum you can charge in order to break even. You will also need to consider how much you need to earn to have the standard of living you desire. When you have all this information, you'll be able to make a decision.