This passage reveals Myrtle to be rather patronizingly proud, as she tells her supposed friend, Mrs. McKee, that she intends to give her the dress she's currently wearing after she's done with it today (as though Mrs. McKee should be grateful for Myrtle's cast-off clothes). By saying that she has to get another dress tomorrow, Myrtle implies that this dress is no longer good enough for her, but she obviously thinks that it is plenty good enough for Mrs. McKee. Further, Myrtle seems to take some pleasure in listing all of the things she feels she needs to do tomorrow, as it must make her feel very busy and special; she wants others to recognize how busy she is, as she feels quite self-important and wants others to see her as important as well.
Myrtle's list of things to do includes several activities that result from her vanity: she wants to get a massage as well as get her hair done. Further, the other things she wants to do involve purchasing things, showing her materialism. Everything is about spending money—Tom's money, presumably—as Myrtle seems to want to feel as though she is of the upper-class. She attempts to be a social climber, and she fails to understand that she would never fit in with that world. This helps to show how deluded she is.