How can you be a famous singer when you dont know how to write a song?

Learn! It's not hard. Just start improvising stuff vocally or on any instrument you play, and sooner or later you'll be doodling and think, 'that could be a great song'. Then it's just working to find the very best notes for it. Or if you really can't do it, find a partner who writes (and possibly plays an instrument) but doesn't sing. Warning; if you become famous, you will get the fame but your partner will get any royalty money.