Hey there, and congrats on wanting to learn the keyboard!
I'm not a keyboard player, I've been playing guitar for 6 years...however all music theory starts from the Major scale, so I suggest you learn how a Major scale is made and what chords it contains.
So start by learning some tedious beginner piano songs that use the C Major scale (all the white keys!)
After you've got the hang of this move onto the Minor Scale which is relative to the Major scale, and most Metal bands favour the Minor Scale rather than the Major Scale.
And in the future you should definitely learn the Harmonic Minor Scale! The most Metal scale of all
Very "Eastern" sounding!
I'm sure someone else can give you better advice than myself but since no one has replied yet you have at least some idea of what to learn.
Good luck dude!
ALSO!!
I'm like 90% sure that keyboard players in Meteal bands de-tune their keyboards/synths to match the guitarists.
So guitarists that are tuned down a semi tone, the keys will be the same (in most cases)
Stratovarius, Sonata Arctica, Firewind do this and I think Children of Bodom do this too.
Knowing this will avoid any future headache about awkward tunings!
