At 18 I had my first child with my ex wife and started my career as a residential electrician. I wouldn't change anything leading up to that.

What I would do from there is to get a 35 year term life insurance policy while it is dirt cheap, since that price remains throughout that 35 years, and invest everything I didn't need for basic living and a bit of fun each month, into a mid-risk Mutual fund.(obviously I'm not weighing in the things I know how, like the status of Bitcoin and other stocks). But setting myself up, and setting my family up for a financially secure future would be number 1 on my list, because now I'm nearly twice that age, and having hit Covid, I had no financial "nest egg" to pull from in the financial crisis, and I've been hurt seriously multiple times from jobs accidents and life accidents, and my family would have been left with nothing. So now I'm just starting my journey into financial security, but I've lost out on the VERY important beginning years that would make retirement very plausible as I grow older and my want for traveling and relaxation grows.