One of the best pieces of advise I can give you is to learn from your mistakes. You can even learn from your pricing mistakes. It takes time to get the hang of things, don't be afraid to make mistakes...you just have to be sure to take something away from those experiences. Its going to be better for you now if you accidently underbid an account, as you are still young. You will soon get a better feel for what pricing should be, especially for your area.
Don't let people try and walk all over you. You may be young, but that doesn't mean that people should take advantage of you and try and cut down your pricing. Stand your ground on your pricing, its not worth making almost no money on jobs. Don't try and be something that your aren't, but understand that even though you're only 13, your services are still worth something.
Keep working hard at this and you will all see your hard work pay off down the road.
'07 Chevy 2500HD
2010 John Deere Z925A 54" MOD
2011 John Deere Quik Trak 647A
John Deere walkbehind