For a light, fluffy white cake, I always use cake flour that has been well sifted. First I measure, then I sift. If you're using cup measurements, you may not be measuring your flour accurately. I always use weight measurements which yield great results every time. If you don't have a scale, or are not interested in purchasing one, then make sure you fluff your bag/box of flour before spooning it into the cup. Be sure your flour mounds up over the rim, then, with a flat knife, level flour to rim. There are other ways to measure flour, but I find using a scale the best way to go. If your cakes are coming our heavy and dense, I suspect your flour measurement is off or you're over mixing the batter. I agree with everything Becky said as well. Creaming should take about 3 plus minutes. For your last step, make sure you alternate flour/dry ingredients with liquid, always ending up with flour. And as Becky said, don't overwork the flour. Sometimes, the recipes for white cake are to blame. Check out Flo Braker's recipe for white cake. You may be able to google it. Good luck next cake!