The best way to cream butter and sugar is with an electric mixer, either handheld model or a stand mixer. You want to make sure you cream the butter and sugar for 3-5 minutes and don't forget to scrape the sides of the bowl. Not creaming the butter long enough will result in flatter cookies that aren't light and fluffy. The longer you cream the butter and sugar, the more air you introduce into the dough, making them lighter and fluffier. Less creaming will produce a dense, chewier cookie (and they'll also spread less).