Are you playing for the first time?
Click on Create a Household or Create a Sim. It's at the bottom left of your screen in the neighborhod screen.
I can't remember now but if it's not available, you might be in tutorial mode. Click on Edit Town and then back. That should prompt exiting the tutorial.
This beginner guide may help:
https://hubpages.com/games-hobbies/sims-3-newbies