As for decorating, practice, practice, practice. YouTube has great instructional videos. There are tons of tutorials here and all over the web as well.
Assuming you want to eventually run your own business and not work for someone else, I recommend if your high school offers business math classes, taking some of those as well. Taking business management, economics, and accounting courses at the college level will also help prepare you for the business side of things.
Good luck!!!
When people ask you to make a cake for them, instead of doing it for free, do it for supplies 
Let them cover the cost of the ingredients and say "I can make that for you, but I need you to buy this special pan" It's a great way to grow your supplies without spending a ton of cash. And people are happy to do it because they are getting a cake they couldn't afford to buy at full price while you get to practice your skills!
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