Sit straight but not rigid, like an invisible string from the back of your neck. Either on a cushion or in a chair but don't prop up your back. These are best but you can lie down as well. Just careful not to fall asleep
Usually I meditate lying down or in a modified lotus. If my teens are home lying down with my eyes closed is ther only way to get 10 uninterrupted minutes
It might depend upon what kind of meditation you do.
I do vipassana, for this this the preferred position would be the spine erect and legs folded
I usually meditate sitting down in the same chair every day. Both feet on the floor. Hans in my lap. Spine erect shoulders relaxed. I am working to transition to cushions on the floor with photos and objects that are meaningful to me as well as a candle in front of me.