1.How to collect response from sim900 corresponding to AT command for call generation
Using mySerial.available() and mySerial.read().
Of course, you don't have a device called a mySerial connected to the pins, so why the instance is called mySerial is a mystery. A name like phone or cell would make a lot more sense.
2.AT command to call no. from sim card is ATD>. But how to use it.
Just like it shows in the data sheet for your phone/modem/whatever.
The AT command is ATD followed by some additional data, not including the plus signs. Unless using the phone manually requires them.