Each adult sim has 3 traits to start off with. There are two ways to get the other 9 traits. The first way is to complete Aspirations. When you first make a sim, you have to pick a Aspirations for them to complete, once completed they unlock a trait that is associated with that Aspirations. You also get satisfaction points while completing Aspirations. Also don't worry about getting stuck on an Aspiration, you can simply switch to another one if needed too. The second way to get traits is to buy them with the satisfaction points. You earn these points though the Aspirations but also from completing the sims wishes. The wishes are the little thought bubbles above the sims picture on the lower right corner of the screen. The points will vary by wish. To spend them, go to your sim's Aspiration and click on the box that looks like a gift. Once you have 12, the achievement will unlock.