Help! i need one help in debugginng one application security

[fsociety]

while i was debugging an app network using fiddler.

i found sig in there post data so i want to know how to decompile the apk and found the sig creation.

sig is somekind of creating signatiure , so wihout decompiling apk no one can say how it made...and i am not pro in doing so anyone good in doing all this...