expected primary-expression before ',' token
To answer your question we will assume your project is based on the Melody example at www.arduino.cc. 

Likely you wanted to enter the ASCII values of these letters into an array of type char (type int would work as well.  But, in some cases, char would be more efficient).  To do this you need to place single quotes around each letter.  Specifically, you need to change your code: 

int melody[]={ a,C,D, a,a,C,D,D,C, C,C,D,D,C, C,D,E,F,E,D,C};


...so that your line of code look like this:

int melody[]={'a','C','D','a','a','C','D','D','C','C','C','D','D','C','C','D','E','F','E','D','C'};