I want to buy another house but have a loan what should i do?

okay i am so new at this. i have a preexisting loan and am wanting to buy a smaller home and dont have the money for the down payment . but i do have enough in the equity of my house that is the cost of the smaller house. how do i go about doing this without selling my home. after i move into the smaller home i want to sell my home and pay the loan off.