Do we need to take Guru Updesha to chant Panchakshari, Dwadashakshari to get complete phalam of chanting?
To not chant because one has not the guru to teach properly is only an excuse by the mind to not do. If one starts and has a sincere heart, the guru will not be wanting, the guru will appear. For every step you make towards God, God makes 10 steps toward you. A mother does not love her child less because it can only say Ma and does not know how to say Mother. She herself will take the child and teach it how to say Mother properly if no one else does. What counts is sincerity of heart. Krishna says in the Gita (9.26) "Whosoever offers Me, with devotion, a leaf, a flower, a fruit, or water-that I accept, the pious offering of the pure in heart."
You don't need upadesa to chant panchakshari (namah sivaya) but you do need upadesa to chant pranava panchakshari ( panchakshari appended to omkaara). No upadesa is needed for naama parayana/sankeertana. Anything with a beejakshara (such as Om etc.) requires upadesa by a guru.