Introduction - If you have any usage issues, please Google them yourself
The idea is that as Fibonnaci values are computed you store them in the array A. So when another F(n) is to be computed either A[n] is still 0 or else A[n] is already equal to F(n).
If A[n] is 0, you have to make the usual recursive calls to compute F(n). Then store this value in A[n] AND return the value.
If A[n] is not 0, that means you have already computed F(n), so just return A[n].