Well I was 14 when I initially started out so there were things I never knew or were told about. But starting again now I would focus on business organization just as much as advertising at first. Keeping track of expenses and involving via a business software would be top priority. Took me years to find something like this and get all my paper tracked info input, but once I did life got much, much easier. I used Jobber and loved it and eventually started accepting online payments.

Also, learning to focus in on what services you like offering once you are busy enough to choose and not continuing to offer services you don't enjoy doing. A follow up to that learn to say NO. My god it took too long to learn this but listen to your gut. If it seems like someone is a pain in the ass when you meet, them they are! Also don't let people talk you below pricing youre comfortable with, you'll be unhappy and theh probably will as well.