How do I recognise/ translate musical notes into sol-fas...?

Ok, i understand what a note is, what a key is, what a scale is...but when I hum a melody in my head or sing the song twinkle twinkle little star... I can't seem 2 translate every note into the tonic sol-fas 'do re me fa so la ti'... I'm not tone deaf cos I can tell when a note is higher or lower than another...pls helpcos I can tell when a note is higher or lower than another...pls help