so after the cake is cooled i put butter cream or any other filling and then i frost it.
So after that do i right away cover with fondant? Do i leave it? Someone on here said to cover in plastic and leave it for 12hrs? i dont understand why do i have to do that and what is the best way?
You need to let your cake settle. Right now, it's full of air. If you put your fondant on now, the weight of the fondant will squish your cake, causing your filling to bulge and the fondant to gather around the bottom.
While you let your filled cake rest, place a weight on it to make sure it's as compacted as it can be without actually squishing it. (several nested cake pans, ceramic tile a la leah, baking stone, e.g.)