Talking more to people
Ok this is more of a personel one, more than what you should do at 18 but I think it would still apply to alot of people, to basically just mix more with people if in college, uni, sixth form or working somwhere. As by mixing with people even if its outside of your comfort zone, it can open alot of doors for you and you may even find new people who could help you progress in life and help you grow as a person. With this in mind the famous quote of don't talk to strangers comes into play whether it be irl (in real life) or over the internet yet the people we talk to on a daily basis or occasionally were once strangers before getting to know them fully.
So being lets say more adventurous could help you to find people who could be a key part of your life as a friend or even end up being your life partner. (Kinda scary when you think about it or is it just me)