Well, it all depends on the sheet. With a point adoptables, you send the points indicated. With cash adoptables, you send the money via paypal. With free, you just ask for it, and they give it to you. With Draw to Adopts, you draw the character and the best drawing gets them.
Adoptables are characters that you own after you purchase them. For example, on my main profile, Kareem is a character I adopted. He's now my main OC.
You're welcome