Click anywhere on the 3D scene to enter it and be able to move around within the world!

Press W A S D keys to move around
Move the Mouse to look around
Hold Shift to move slower
Press F to toggle flashlight
Press Esc to get mouse cursor back

Click or Click and drag to select polygons
Double click to select the connected polygons of the same texture
C to clear selection
Del to delete selected polygons
Hold Shift while selecting to add to existing selection
Hold Ctrl while selecting to remove from existing selection