Good question, I’m left-legged in football Well, for me this is a cultural aspect, because in the eastern culture- lot of it is derived from Islam- the right is considered as clean and proper, it’s Kinda Protocol. so we use the right hand to : Eat ,pick up clean stuff ,to shake hands ( never enter your right fingers in your nose ,no one will shake hands with you ,hahaha :-) ) ,...etc. And right leg to: enter house, to enter mosque (like church for Christians), first one to enter in the shoes...etc. The left hand is used generally to give something like shoes, dusty or dirty stuff, and used in Bathroom… Peace Out ;-)

"When starting to walk, why do people use their right leg first?" And I disagree...I usually use my left leg first. (and I'm right-handed) I think you step with the leg that is not your main "balance" leg first to keep yourself firm on the ground...but I'm just guessing.

You should probably phrase this as: "Why do people walk using the right leg first?" and I have no clue

It depends on how your parents teach you.