A good meditation posture refers to the Seven-Point Meditation Posture of Vairocana.
#1: Sit cross-legged. You can choose to cross both of your legs or just one or sit with your legs spread apart.
#2: Keep your body upright.
#3: Form the Meditation Mudra, with your right hand resting on top of your left hand.
#4: Extend both of your arms outward, rather than inward.
#5: Slightly lower your head.
#6: Unless in special circumstances, gaze downward at the tip of your nose. Keep your eyes slightly open to allow a bit of light to enter.
#7: Place your tongue against the roof of your mouth.
Generally speaking, during meditation, you should relax every part of your body, without any tension. If you don’t sit upright, you can’t relax. It’s like stacking things—if you don’t stack them straight, they will fall apart as soon as you let them go. Only when you stack them straight will they not fall apart. The same goes for sitting—if you sit upright, you can truly relax. In this way, your mind can calm down and engage in meditation practice.