Be kind to yourself. Happiness is the part that I wished I'd appreciated more when younger, in ruder health, fitter and slimmer. (Admittedly, I'm struggling more now than I might not have done had I never had a head injury and post concussion syndrome before being injured permanently by an off label antipsychotic, but that is another long story....)
As the Sunscreen song so aptly says, enjoy the power and beauty of your youth. Dance, floss, look after your teeth, your mental health, your bones, your brain, your mind.
Don't bang your head like i did (!) Try to minimise medication unless absolutely necessary, especially psychotropic drugs, as these are absolutely horrid for long term effects (and i should know 😫💔)
Exercise properly, appreciate your slim size 10 waist now, and try not to gain weight around perimenopause. It'll take much more effort to lose the middle age weight than you'd ever anticipate.
Be kind to others, be empathetic, and don't suffer fools gladly nor tolerate someone being unkind. Often, others' criticism or unkind words are actually a reflection of their own feelings and shortcomings.
Marry a kind, practical, reliable, and good man. Yes, even the bald one who looks a bit like your dad and reminds you of a kind adult friend of your parents who made you feel safe and comforted you (DH, I'm looking at you... 😅) Really don't make do with soneone who's a great friend and a laugh, but who you don't fancy, and is more like a brother. And definitely avoid the men whose mums are racist old battleaxes who are slightly scary...
Oh, and save money, pay into a good pension, work for a good company, and never lose the connections with people who employed you. It's usually who you knew that helps you get on in the later stages of your career.