Start at red buff and as soon as you reach lv 3 try to gank. You can use exhaust to help. After that go for another gank and lv 5-6. As soon as you buy your 3-rd Item (Frozen Mallet), sell your Philosopher's stone and buy Tiamat. You can farm with it easier. Don't take much risks.
Don't forget, that in team fights you should attack from the flank and target the weakest(with less hp and armour). I guarantee you that you won't fail ;)

This is not one of my best builds. I mean it is my first one made.