I don't have any idea how to add leaderboard and Achievements in my game .please anyone suggest me how to add leaderboard and Achievements in my game .

Moving you to a more relevant forum.

What platform is your game being developer for? Various platforms offer their own Leaderboard implementations.

If it's a online game, you can use facebook for this.

+ Easy to do with both Unity and Unreal. Also has tools for custom engines; very easy to use.

+The nice thing is you won't need a server. 

+ Can keep score and achievements.

- The downside side is that players can only view score of friends they are playing with.