Instructions
Choose an apartment lot for your single Sim or household. Select the door of the empty apartment where you want your characters to live. Click the door and choose the "Rent Apartment" option to move in.
Find a roommate to share the rent by using a newspaper or a computer. When you try to find a roommate this way, a list of potential candidates will appear. This list will include each candidate's personality type and job skills. Your roommate will move in immediately after you have made your selection.
Ask one of your Sim's friends to be your roommate if you would prefer not to have a random character move in. Click on the potential roommate, choose the "Propose" option, then select "Be Roommate." Selecting "Move In" rather than "Be Roommate" will add the Sim to your household and let you control it, which also means you will still be responsible for paying the entire rent as well as taking care of an extra Sim. The new roommate will move in immediately.