-Be careful with your sexual health and contraception (get tested regularly if you're sexually active etc etc).
-Research the career you want and ways in; you may have to volunteer or make a sideways move and you may not end up where you thought you would so it's good to try a few things. If you have no idea; temp, volunteer, try as many roles as possible, talk to as many people as possible to get ideas.
-Lots of pp have said and I agree: say yes to opportunities to go out/socialise. It's the easiest time to meet people at that age, whether that be friends, partners or even just people who might spark an idea for you. Have lots of late night conversations with people about life, the universe and everything; I miss that from my early 20s! (no-one seems to want to anymore)
-Don't waste time in a bad relationship, don't move in with a partner you're not sure about but equally if you do meet someone amazing, ignore people who say you 'have' to be single in your 20s to experince things; it really depends on who you meet and what happens, either way is fine.
-I agree with the pp who said save; even if just a bit a month. Set up a regular payment to a savings account; that way you have a cushion for anything going wrong or if you suddenly want to move to a new city or whatever, it gives you more options.
-For me personally, this was the time I started thinking the stuff from my childhood (divorce etc) to make sense of it all; I was getting anxious due to some issues with some of my relationships that I needed to work through. If you feel like this; counselling can be a great way to work through some things. Consider it an investment in your mental health. My relationship with my family is better now because of that I think. I also talked to my DSis about stuff and our relationship is in a different place now.
Sorry for the essay, ignore if none of that applies to you!