I am 23 'years old an I have a problem of ejaculating very fast mostly during the first 10 seconds of time. I have been masturbating but five times in an year, how can I increase my time up to 20 - 30 minutes?

There is no premature ejaculation when you masturbate. Just stop before you pop!

You might want to research kegel exercises. But before actually performing them, consult with a therapist.