Can you take small dogs on a plane and keep it under you seat? if so, how much money?

how much does it how much does it cost to carry on a small dog? i am thinking about getting a new ppug puppy and am traveling to hawaii for a week. what should i be prepared for overall ( including when we and the dog are in hawaii)??? and can i simply store it under my seat in a dog carrier? also, is it true that you can take it out when the seatbelt sign is not on? how difficult would it be with a puppy? thanks, anything helps!:)