Metal is entire genre that is full of so many subgenres, it would take a while to explain each. But in general, you can go from fast to slow, heavy to soft, vocal to instrumental...
As an intro to Metal, I'd start with the classics. Some of the first Heavy Metal stuff of Black Sabbath or something, or a little after with some Thrash and Metallica or early Prog with Dream Theater or Queensryche [two different forms of Prog]. For me, though, I think the really interesting stuff starts off in the 90s. Death, Black, Symphonic, Progressive Metal really starting to come to life. Or play it safe with some "Alternative" Metal, which is basically just Rock...
Once you get through the 90s, you start getting a lot more weird, and into more territory of "that's not even Metal!". Things like Metalcore, or a third style of Prog often named "Djent".
Also some particularly lovely Technical Death Metal. Or on the slower end, Brutal Death Metal... This is where you can see microgenres really starting to come into place. You think subgenres are difficult to follow?
Seriously though, this is a genre that spans so many things, that you can be sure to find something.