Usually store bought cakes are flat and homemade ones are fluffy. Follow the recipe properly. Keep all ingredients ready before u start and at room temperature. There are many things involved if a cake is flat..oven has to be preheated, dont beat the batter too much, when dry ingredients and wet ingredients are mixed soon it has to go in the oven. N low fat cream wont whip, for whipping cream fat has to be above 40%. U can also try buttercream icing or cream cheese frosting if u dont find whipping cream.