I kind of a new player and I would like to begin combining heroes from different spheres.

My question is: can player deck cards from one spheres be played with a hero of a different sphere? For example, can the Unexpected Courage (Spirit) be attached to Aragorn (Leadership) as long as it paid by resources by a Spirit hero?

Thank you!

Yep, generally. Unless the attachment specifies a sphere restriction (e.g. A Burning Brand, which can only be attached to a hero with a Lore resource icon), it can usually be played on any hero (and/or any ally, depending on the card text).