I'm not a building expert, but I've added second stories a few times.
First of all, you need room on the ground floor to put a staircase. They take up a lot of space, which you'll see when you try to place one. Sometimes the best place is next to a wall. You have to remove all objects that are in the area where you want to place the stairs. Remove all furniture, light fixtures, pictures, everything. Once you have enough space, go to Build Mode and click on the stairs of the house diagram. Select which kind of stairs you want, whether you want the rails, and how wide you want it. It's best to have it a minimum of 2 wide because that allows 2 Sims to use the stairs at the same time and avoids pathing problems.
You'll have to delete your roof to allow for the second story. Once that's done, you build the walls of the second story. Use the level up button to put you physically at the second story. You can put the walls exactly where the ground floor walls are or you can build them in other spots to allow for a balcony. After the walls are built, put the floor down. You can also put a fence around the opening of the stairs. Sims won't actually fall down that hole, but the rail around it is pretty.
After you have the second story walls and floor built, put the roof back on the house. You can either use the auto roof function or you can put it on manually yourself.
Here's a tutorial on how to build a multi-story home.
Making Homes with Multiple Stories and Basements
https://www.carls-sims-3-guide.com/buildedit/houses/multilevel/I hope this gets you started. Come on back if you have trouble and we'll try to help you.
