In your SH3D you should have a piece of furniture called Box, select that and drag it in your project. Then you can change the size and height to fit your room.

In case the image broke: https://sta.sh/01138nje06xy

For baseboard, click on the wall you want to add a baseboard, then go to 'modify baseboard' then modify as you'd like.

In case the image broke: https://sta.sh/025aj598pp2k