Welcome here trod.
You've done well to get yourself such a great camera!
The instruction manual is your friend - read it through well. It helps you navigate through the reasonably easy to use and intuitive menus and functions.
Any K-mount lens will just fit straight on the camera.
For any pre-A lenses (ones without an A setting on the aperture ring - if it has one), you must ensure the camera is set to allow non-A lenses to be used on the camera (check the manual for the actual location of this setting).
Other commonly used lenses on Pentax dSLRs are the M42 screwmount variety, which need an M42 to K-mount adapter to be fitted between the lens and the camera. This gives you the ability to fit those vintage lenses onto a sophisticated digital camera body.
As for a 50mm lens, Pentax have one of the best fast fifties in the FA 50 f/1.4. Relatively cheap (around $200, even less second hand from this forum's marketplace

, it is one of the best performing low-light lenses available. If you like manual focus lenses, then you can get just as good a lens for much cheaper (like the A 50 f/1.4 or f/1.7 versions).
All the best in that.