How do you make a very fluffy light cake?
What is the ingredient that does it?

My cakes are always heavy I can't make it like the ones in the shops.
Anyone have any ideas?

A problem might be with your location. Cities in high altitude (2000 ft or more above sea level) will require you to adjust most cake recipes that were made somewhere with 0 - 2000 ft altitude. So look up your city and check your altitude.

Also, people swear by WASC or whatever. I've tried it multiple times, I don't think it has every worked out for me and I really don't care for it. The cake recipe I use is the CIA's High Ratio White Cake. It is fluffy enough without compromising structure.If you are interested, message me and I will type up the recipe for you when I have a chance.

One way to lighten any recipe is to use the chiffon method. Using all the same ingredients and amounts already present in your recipe: separate the egg whites from the yolks, whip the whites to medium peak with half of the sugar, make the rest of the batter as usual with the yolks, and fold the egg whites in at the end. Instant added fluffiness.