Figure out how long it will take you to mow a certain amount of grass. Then consider what the going rate is for teenage workers in your area. For example, if the going hourly rate is $10.00 and if you reckon it will take you 30 minutes to do the job, charge for half an hour, $5.00 and so on. Remember you should really round up your price for small jobs because they take just as long to get to and set up.

Another source of info would be to look at other teenagers' ads. in the local newspaper or store windows.

Hope this helps.