The conglomerate of all of your attributes forms your personality. It also delegates you to an astrological sign. For example, an excessively neat person will be a Virgo. A well-balanced Sim will be a Cancer, and so on. You can also create a Sim according to what sign you'd like to be.
Specific signs get along with other signs in various ways.
Once you have assigned a personality, go to the right panel to design how your Sim will look. Toggle between adult and child with the top tier of buttons. Choose the skin color of your Sim by clicking on one of the hand icons. Decide whether your Sim will be male or female by selecting the appropriate symbol. Then move to the character display to cycle through the various clothing and head options.