Watch the Learn the Basic Chords online guitar lesson by Fareed Haque from Solo Guitar Handbook
OK, when you're done wiping the tears away and the giggles have subsided, then you sing/hum/croak the melody and try to feel where the chords should change. Usually, it's on the beat and the chord falls on an accented or important word. Use simple chords. Use your instincts and they will develop.