how did Какаси get the scar by his eye?

I thought he got it when he was younger and just became Jounin. This answer might have spoilers depending on where you're up to so read at your own risk xD lol

If you've watched "Kakashi's Gaiden" (episode 119&120 i think), he was on a mission during the third great war and him, Obito, Rin and Minato (4th hokage) were on a mission to destroy a bridge to prevent enemies или something but Rin was captured by a few enemies wanting information and Какаси and Obito went to rescue her. While rescuing her, Kakashi's eye got slashed and he was blinded but then Obito who was crushed under a rock, gave Какаси his eye which had the sharingan in it (Rin did the medical transplant).

The scar was most likely from when the enemy slashed his eye, and he would've Остаться в живых an eye/being half blinded if Obito didn't give him his sharingan. Hope this answer helps :)

DUHHHH......Orochimaru Gave Him It By Giving Him The Sharingan With The "Otakura, Moon Fall Shadow Jutsu"Wit His Sord!!!!!!!

a person scarified it in a battle.

Long story. Just watch the video you'll find it at
link