I would say:
Career
Earn as much money as you are capable of making, in a job you enjoy and ensure you pay into a pension. (I spent too long faffing about in dead end jobs wondering what to do with my life. I had several talents I never used and was often penniless.) Be as confident and ambitious as men. Ask for salary raises and career development and skills training. Move jobs every 12-18 months until you are at a salary level that really appeals. Apply for jobs that are just outside your scope of experience, not just those that are well within it. Men do. That's why they get them.
Health
This REALLY matters. Stay fit. Work out. Focus on health and strength and stamina not thinness or the latest weird body shape crazes. Find fitness regimes that you love and stay active. Look after your eyes, teeth, and your mental health. Like PP, I had ADHD and didn't know. Nothing could be done about that back then as it wasn't recognised but if I had my time again on the correct medication, my life would be unrecognisable. If you have mental health issues, sort them. Invest in yourself.
Spend money on what matters. Don't fritter it on fast fashion, boozing until you puke. Make a list of things you really want to do and own, and save for these and buy them.
Have adventures
(I did this. A lot. That was one thing I definitely got right.) I said earn a fortune but that doesn't mean be a wage slave. Look for chances to work abroad or in a profession that really excites you. Take adventurous holidays. Take on challenges while you are free and single - from doing stand up comedy to singing in a band to training for a triathlon. I am SO glad I had a load of adventures. It meant by the time I had children they were the new adventure. I didn't feel like I'd missed out.
Just accept this truth: Youth is beauty and beauty is power
Don't waste one second of your life thinking you are not attractive enough or in any other way not good enough for some flaky man/ manipulative boss or friend or family member. Chances are you are gorgeous. If you are actually medically overweight you can lose weight and tone up and your young skin will spring back. If you are medically within healthy BMI you are not fat. Don't choose to believe you are. I say this because so many women waste so many years of life hating themselves for the slightest curve of flesh. Just don't. You'll only look back at forty, fifty, sixty and think: wow, what was I thinking? She is so lovely.
Walk through the world as if you are gorgeous. Set high standards of respect and kindness from friends, partners, and colleagues, and you will attract the right people into your life.