Click on an empty house in the neighborhood section and choose one to build (including the empty lot, if you want your new Sim to live there). The only way to add more Sims is to build a new lot/house. When it is completed, click on it again, and you will get the create-a-sim section.
I have a big piece of advice for people starting out in the game (as you are). If you ever plan to spend real money to purchase Simoleans, do it at the beginning of the game as everything gets more expensive as you go along. By the time you reach level 20, building anything costs huge amounts of Simoleans and unless you plan to spend $99 real dollars your Simoleans won't be worth squat! In general, I would advice buying Life Points (LPs) over Simoleans, as they are much, much harder to acquire, but at the beginning of the game it might be worth it to buy Simoleans and always be building lots - businesses and shops as well as things like The Mansion. By the time you earn enough Simoleans to build the Mansion is will be 5 times (loose estimate) the price it is now! Look online for a list of the building prices as you progress through the game. Also, even though it is the job that pays the least, don't wait too long to build the Firehouse! You will get a goal to employ two firefighters and I got stuck for awhile there because I had build every workplace EXCEPT the firehouse!! I would build 1) Science Center (highest paid) 2) Firehouse and then look at one of the online charts to see what the next highest paying job is.
Hope this helps!!