Happy Birthday OP
Man there's hella shyt I wish I knew at 18 that I know now
If you just turned 18 I assume you recently graduated from high school this summer too? If you don't know your career path yet I would strongly advise you to take a year off from school or to just attend a local community college and get your basics done. There are plenty of people who get pushed to go to Universities and don't even know why they're there and are accruing unnecessary debt. Take some time and reflect on what you really want
Your social circle post high school will change dramatically and you will outgrow people if you haven't already. There were plenty of people I thought i would still be rocking with now (I'm 25) but I fell off with most of my high school friends. Mindsets and goals will change.
Don't get any women pregnant. I cannot stress this enough. I've seen unexpected and unwanted babies ruin the course of many young peoples lives. Plus baby mama drama is always a headache.
Start saving if you haven't already. Get into the habit of tracking your expenses. There are plenty of apps to help with that. Financial knowledge is something a lot of young people are lacking.