As someone who recently turned 21, I seek guidance from some of our veteran members of CB.
What do I absolutely need to try?
Everyone has a different holy grail, but here is my advice: Take baby steps.
If you're like me, when you turned 21, your own experience with beer is mostly limited to whatever people give you, and when you actually do get to tell someone a thing you want, you didn't know enough to actually select anything really 'good'.
To me (and I'm kind of particular)I'd start by trying very common and cheaper examples of 'types' of beers.
Killians Irish Red
Yuengling Black and Tan
Harpoon IPA or Sierra Nevada Pale Ale (they're basically the same)
Sam's Boston Lager
Guinness
Smithwicks (pronounced 'Smiddicks')
Those are all baselines..they're not the 'best' examples of their genres, but beer is kind of like wine. Until you know what you're drinking, you're probably not going to appreciate it as much as someone who has refined their pallet, and the smaller and more unique the beer, generally the more expensive. Its best to find out what you like before you go spending $10 a bomber (a 22 oz bottler of beer).
If I would've tried like say...
Dogfish Head's Burton's Baton Imperial IPA when I was just turning 21, I'm not sure I would've liked it. If you get my meaning. Find out what you like, and then explore.