As meditation requires you to sit still for a while it is important, that you can sustain your posture without distressing uncomfort. If you are a beginner in meditation you should not attempt to sit in the full lotus posture with your feet lying on your thighs because it will make your hips hurt from the very beginning.
Generally it is necessary to sit upright with a straight spine, not leaning on a backrest. You can sit on a chair or a thick cushion in a kneeling posture. As your body is not used to sitting without movement and even the kneeling posture is new to your knees you have to be patient with your body and extend your meditation sessions gradually.
Especially as a beginner you will be very much distracted with all that kinds of pains and aches in your body. In my opinion it is better to seat your self as comfortable as possible (still with the straight back…) than trying to be brave. The more you get into meditation, the more you will just notice your sensations without being distracted by them.