Many people ponder what they would advise their younger selves. That assumes some extremely unlikely things beginning with time travel and including young people taking advice from elders. But these flights of fancy push those aside for purposes of exploring how life’s turns work… or coulda.
Make friends with Colonel Sears (the neighborhood ham radio operator). Hams in the ’50s, ’60s and ’70s are ahead of what will become an explosion in electronic technology. Learning about the world of amateur radio will provide a foundation for a massive world of electronics to come.
Take Karate lessons… assuming you can find them. Self-defense will always be a handy thing to have. Karate teaches much more including self discipline, control and respect for your body.
Help a builder. Find someone who builds houses, plumbs them, runs the electrical or all three. Work for them any way they will let you. Learn these fundamental skills. The coming building boom will treat you well. The next decade of stick-and-ball games will be amusing, but in the long run, nearly useless.
Buy a Conn 6H. Through the 60s, these were among the best mass produced trombones ever! This horn would never hold a trombonist back, and could last a lifetime. Your next horn(s) should be a Conn 8H and an 88H. The pre-1970s horns were great.
… on a related note …
Take lessons from a good trombonist. Get the foundation right. Build on good technique. Don’t spend 40 years making the best of bad form.
… and furthermore …
Do not spend more than two school years in marching bands. What a waste of potential musicians! Six years I spent in public school “advanced band” classes. Oh could we march. Ugh. Shop classes, art classes, and so much more were missed so we could put on a show marching and playing a horns simultaneously. Be eager to play your horn outside of public school situations … wherever, whenever, forever.
You ARE good at math. I don’t know how you got it into your head that you aren’t, but that is wrong. Be comfortable with it. Advance in it. Mathematics are great foundation for what you ought to be studying later. The history taught in your schools is half-truths at best, but the math is solid.
Buy a ’62 Chevy II with a factory 4-speed. I know you are only 13 years old, but figure it out somehow. So much could be done with this car as a foundation, beginning with learning to drive a well-build light-weight semi-econo-car. Find a mechanic who will let you help with the maintenance of it, developing more priceless knowledge and skill.
Join a gun club. Learn about firearms, their qualities, how to maintain and how to shoot. These are ‘forever skills’ that everyone should have. Plus you will get to know some really good people and learn a lot from them in other areas too.
Pursue electronic engineering. Cal Poly, San Luis Obispo would probably be your best choice. You will be at the leading edge of an explosion in this technology. It will challenge, amuse and pay you well. By the way, Bill Hewlett and Dave Packard will be among the best employers EVER, particularly for electronic engineers.
Join the Sports Car Club of America. The kind of driving you were born to do is what they do. You might as well discover your kindred spirits and enjoy the heydays of sports cars. Don’t sell the Chevy II to go racing. Buy a race car for that. Porsches are good. So are BMWs of your era. Stay away from British cars.
Also stay away from motorcycles. Your high-performance driving style punishes motorcyclists harshly for mistakes and even more so for the mistakes of others. Motorcycles and tobacco are two things best not started on.
Join the Libertarian Party. You will have to wait until it is founded in 1972, but your real education about how the world works won’t begin until you find these folks.
… through them you will find …
Study Austrian Economics. Not to become professional. Nobody will hire an economist who doesn’t support the popular mythology. Do this to understand Human Action, how a healthy society works and to easily recognize failed models.
That was my little fantasy. 
It is not about regrets. I believe that we are where we are by good fortune. Had we taken a left instead of right at some fork in the road, we would have been hit by a train and not made it to our current life situation.
Whatever is in your past, enjoy it for what it is, and keep it there – in your past.
Today is the only day you have, but betting on tomorrow will always pay off
… until it doesn’t, but that only happens one time per lifetime.
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I will shortly publish a similar essay, but design it for today’s youth. Much has changed. Some has not.