How to connect to an SAP module?

You can create RFC function module and then call this function module from outside. You can create RFC using T-Code SE37

You can use the following link to know more about using RFC function module:

https://archive.sap.com/discussions/thread/333645

This tells about how you can create an FM in SE37 in Target system enabling Remote-Function Enabled and using attributes of FM.

To create an RFC connection, you need to use T-Code: SM59 mentioning Username and Password.

Another link that you can refer which tells about creating RFC and Remote-enabled FM and call from another SAP system using ABAP Program:

https://wiki.scn.sap.com/wiki/display/Snippets/Creating+RFC+and+Remote-enabled+FM+and+call+from+another+SAP+system+using+ABAP+Program