What to do for fasting on full moon? From this coming full moon I've to start fasting and keep it on, so what I've to practice on this day ?
Visakha Uposatha Sutta AN 8.43  and  Muluposatha Sutta: The Roots of the Uposatha AN 3.70  are good reads in this regard. Also in the Uposatha Sutta— Practicing uposatha —AN 3.71 the Buddha defines how lay people should practice Uposatha and describes the different types of devas.

Uposatha practice is the practice by which lay people rest from their daily work, devoting a day on the full moon, new moon, and quarter moon days,  to listening to the Dhamma and training the mind in meditation. The question is whether one does it the proper way or not. The practice of taking the eight precepts in the morning, then taking the five precepts again in the evening, is not proper. In the Uposatha Sutta, Buddha describe it as ‘the Cowherd’s Uposatha.’  It is not the Uposatha of the Noble Ones. The eight precepts should be observed for at least twenty-four hours. 

Another thing to be mindful about is that those persons who, on uposatha days plan to go to quiet places to meditate they inadvertently fall into other trappings. They at times end up in the company of friends and get into other unrelated conversations, because they have no control over their minds. Even in meditation even when they attempt to direct their minds to the samatha (calm) or metta (loving-kindness), their thoughts get scattered aimlessly and they end up only with the visible manifestation of the recitation. The 24 hour uposatha is for the mind, and not for the body. Lastly, it is important for one be mindful to take the five precepts to formally end the uposatha, whenever one wants to terminate it.