How would you tell a friend who don't believe in God that there is God?

What you've said is perfect. Your life, actions, and personality right there speak for God. If the your friend ( we'll call him or her friend A ) walks with Christ then friend A's actions will speak much louder than any words can. Many people don't like to be told " You know that is wrong and you should at least read the bible blah blah go to church walk with god" they are quickly on the defense because they feel we are judging them and criticizing them. The best thing friend A can do is be a friend to ( friend B ) and if friend A happens to be going to church or some sort of gather kindly say " I know you don't really believe their is a God but I just want give you an open invitation to come to church with me sometime" This way friend A is not being rude about it or abusive and leaving it up to friend B to decide when friend B is ready to make the first step. Best of luck to you and God bless.