When I turned 21, I didn't know beer much at all either. I got recommendations from a friend of mine (who also introduced me to homebrewing) to buy the following:
Deschutes Mirror Pond Pale Ale
Deschutes Black Butte Porter
Pilsner Urquell (Or Trumer Pills if you can get that)
Edit: I guess I got some Grolsch as well. I think the porter was on the counter. I was sort of into being a beer nerd before I was even into beer.
After getting familiar with those, he had me pick up:
Young's Double Chocolate Stout
and I think an IPA as well to test out, since I hadn't had one yet.
Most of these are widely available, so see what you can find. Be sure to read up on what the style of the beer is before you try it to really understand what you're tasting (rather than just remembering "oh I don't like Guinness").
If you're really into having a proper experience here, see if you can get some proper glassware as well. Three pieces should be fine: A pint glass, pilsner glass, and wheat beer glass.
Wild Love Brewing
Brew Stats | Brewlog | Fermentation chamber
Drinking: Late for Breakfast Stout, How Rye Am I? Saison, HD 2013, Graff
Bottle conditioning: Nut Brown Ale, EdWort's Hause Pale Ale
Primaries: Dry Irish Stoat, Oatish