From the menu bar, choose File and then New System. You'll see the opening screen of the New Body Wizard:
The New Body Wizard literally steps you through the process of adding a new body to your system, and is pretty much self-explanatory. Here are the kind of screens you can expect to see when adding a single body to a new system.
Things are slightly more complicated when you want to add a second body, and you are asked whether you wish this body to be in orbit around the first, or whether it is moving with respect to the first body. If you choose it to be in orbit, there are then some steps which guide you through specifying the kind of orbit.
When you are finished building bodies, click on "Done". You will be given the option to save your new system to disk, and then you can run your simulation and see what happens.