How do I get rid of my lisp?

When I say my "s", it sounds more like a "shh". My friend even mocked that is sounded like a rain maker when I hold my s. I tried to get rid of it by days of practice by talking with the pencil in my mouth, but it didn't work so well.. And also, i realised that my right side was ok with the s, but it was on the left side of my tongue that doesn't let out a sound, and when i try, I can hear a worse lisp.. Any help?