Perhaps the area where you will make the most mistakes when you start a new business is in marketing.
Advertising is expensive and a mistake here is a costly lesson.
A few years ago I was running a typing service and decided to try a classified ad in the newspaper. The ad ran for 1 month and generated enough revenue to just pay for the ad. The majority of the business that I received from this ad was for typing resumes and the customers did not require any additional work. It was not cost effective to repeat the ad for a second month. This year we ran an ad in a local publication that we thought fit our pool service customers perfectly.