If the network traffic is in "plain text" (not encrypted) then a network sniffer such as Wireshark is ideal.
If the network traffic is encrypted via SSL but you have the private key (or if the client application does not validate the server's public key) then you can use a proxy such as Burp.
If the network traffic is encrypted via SSL and you don't have the private key readily available and the client validates the server's public key then you can use a program such as Echo Mirage, which hooks Winsock's send and receive functions, thus giving you access to the plain text before it's encrypted and after it's decrypted.
Alternatively, if network traffic is encrypted using something other than SSL then you'd need to reverse engineer the client software to determine how the encryption and decryption are performed. Your best bet would be to find cross-references to Winsock's send and receive functions and determine how the client encrypts the data before sending it and decrypts it after receiving it.