Look at the properties for the Single Source Layout, (Click View > Pods > Single Source Layouts) Then double-click the layout. Depending on what your output type is, there will be a button somewhere in the properties that allows you to nominate the default topic.
If you can post back and tell us the layout type you are creating, we can provide better info.
You might also want to give the link below a read.
Helpful and Handy Links