How can I change the type of extrusion output in Siemens NX 12?

I´m new to Siemens NX 12 coming from Fusion 360. I´m having problems with the extrusion command in NX 12. I´ve created a sketch where I want to extrude one big body and then add with the second extrusion some parts to it. When I select the lines of the sketch and then look at the output of this operation I only get walls with no thickness but not a solid body. How can I change this behavior?

thanks for your help 🙂