Sharing and getting scores online, leaderboard

Hi , i have just completed my first game in unity and uploaded it to google playstore named GravityBird, https://play.google.com/store/apps/details?id=com.Shoaib.GravityBird

now i want to save scores online just like in games such as subway surfer it shows highscores of your friends and random people which have the same game whether they are on facebook or not, someone please give me any direction how to start with it, any guidance, even if its different or tough, im ready to start from scratch. Any help would be appreciated thanks.