Happy 21st and enjoy the wonderful world of alchohol you now have access too.
Remember, only drink what you enjoy drinking and NEVER just drink to get drunk, it's a waste of time and good booze!
Since you're just getting started I would avoid the beer. Wait until BBQ season and then pick up whatever folks around here recommend on a nice, hot summer day after a good steak. That's the time for beer and once you acquire a taste for it, you can drink it all you like.
However as your FIRST ever real Alcohol Purchase I would recommend some Captain Morgan's Spiced rum. Mix with some coke in a tall glass until it tastes good to you and enjoy. A Rum and Coke is a good all rounder, it actually tastes good and has enough punch to see you through an evening. Similarly, Vanilla Vodka and Ginger Ale has the same effect and is just as tasty.
If you want something "straight," Peach Schnapps is tasty when sipped and Bailey's is very easy drinking.
I would avoid shots, since you're new at it, and holding the alcohol in your mouth longer than you're supposed to can be unpleasant.
Again, since you're new, avoid Whiskey, Gin, Rye, Brandy and anything powerful that was meant to be had "straight" or "on the rocks".
If you want to try Martini's I recommend going out to a restaurant or bar that does them well, learn from the best before you try it yourself.