I agree with others here: manifestation is nonsense. Or at least it's a sloppy way of describing the idea of careful planning and hard work, but without that careful planning and hard work. Wishing for something to come true without doing the hard graft is a losing proposition.

On the "things I wish someone had told me back then":

• Start investing in index funds. Steady saving as early as you can on low-fee investments with more stable returns is the way to go. It's not as glamour or exciting as day trading, but it's the safer and smarter approach. Keep more volatile trading limited to a limited budget of 'fun money'. Treat it like gambling on horses or playing poker - assume you will be flushing that money down the drain, and once you've hit your fun spending budget don't go behind it, even by a penny.
• Don't take your health for granted. Find a sport or exercise you really enjoy and lean into it. Soccer, basketball, martial arts, gymnastics, going to the gym, Street running, cycling, baseball, or whatever. You don't have to be good at it, you just have to enjoy it. Your proficiency will increase with practice.
• Watch your diet for too much sugar, salt, ultra-processed foods, etc. When you're in your early 20s you'll probably feel like you're indestructible. But the decline without watching your diet (and without regular exercise) is slow but insidious.
• Take time out to just connect with your environment, friends/family, and yourself. Connect with nature. Practice mindfulness and meditation. Do yoga (that shit is deceptively amazing!).