Start building credit as soon as you can (annoying and not something I support morally, but unfortunately necessary).
Establish a primary care provider, dentist, and therapist/psychiatrist as needed.
Put 10% of each paycheck into a savings account.
Put aside at least 50 dollars a month for fun. Treat yourself. Seriously, it's important.
Buy a regeant test kit if you're going to do drugs...make sure you're getting what you wanted, read up on all possible interactions, take the proper supplements, etc. And do the drugs if you want to! Drugs are fun!
If you qualify for EBT, rent assistance, or any other kind of financial assistance, get it! Maintain it!
Take night drives and blast music. Scream-sing at the top of your lungs.
If you have the means and want to adopt animals, do. Hide that shit from your landlord, most places won't find out or care (though I'd ask other residents to make sure).
Attend college if you have the means--not necessarily for the degree, though that's cool too, but to meet other folks your age. Hell, don't even attend if you don't want to, just show up on campus and hang out.
Prepare yourself that all relationships end. Even the most beautiful, enduring, healthy relationships come to and end. It's very rare that they last forever (though anything is possible!)
Be weird. Go to punk shows even if you hate that shit. Volunteer at a shelter. Go to clubs and get wasted. Hug your friends.
Save money, but remember that it exists to be spent.