Ensure that the active Printer and Profile are the ones that you want to be active. (To change to a different profile, you need to click on CUSTOM)

The Tools along the left edge of the screen are used for importing and editing your model (scale, orientation, etc)

Experiment around with importing an STL file and manipulating it in the build environment.

Right click and hold to rotate/orbit, Middle click and hold to pan, middle scroll wheel to zoom.

When the model is yellow it fits in the build area and can be printed. If the model is grey, it does NOT fit. Red areas on your model are overhang areas.