In my first attempt to make cupcakes they came out more like muffins. This in part was due to my lack of creaming the butter and sugar. This is one of the most important steps in making cupcakes. This step allows air bubbles to be created which results in a light and fluffy cake.
These two links I’ve found extremely helpful. The first is just text on how to correctly cream the butter and sugar. The second is a video in which you can see what correctly creamed butter and sugar should look like.