< This instructional video says it all very comprehensively regarding the position of the guitar itself.
When it comes to seating, the main thing to remember is to sit up straight, but keep your posture relaxed, not tense. Make sure your body does not feel twisted and/or hunched over, as this will become very uncomfortable and can lead to injury.
Your feet should be flat on the floor. If it feels like the guitar will slip off your lap, you need to lower your chair, or get yourself a footstool for the leg the guitar is resting on.