ADP is formed from AMP. Adenosine monophosphate (AMP) is a nucleotide. There are many chemicals synthesised inside the cells. There is continuous synthesis and use are going on with in the cell. Chemical reactions are continuously occuring inside the cell.It is these number of processes which generate all the chemicals needed for the cell to function.
Formation of nucleotides needs Pentose sugar(ribose), N2 base, and Phosphate group.For Forming AMP ,N-base is adenine. SO Adenine,Ribose sugar and Phosphate group are neded to form AMP which inturn form ADP by taking another phosphate group.
So many reactions keep occuring,for your level those details are difficult to understand.