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Hi Roc,
Developing for Liferay Commerce is really no different than developing for Liferay DXP.  There are different extension points, but the patterns and frameworks are very similar so if you are new to Liferay development in general, I recommend you start by looking at the DXP development guides and tutorials for your version of Liferay and then move on to the Commerce Development Guide. 
Some additional information would also be helpful:  
Which version of Liferay and Liferay Commerce are you working with? 
Do you have access to the source code developed by the external company?  Which Commerce modules are you trying to modify?  And were they already modified by the external company?  

Hi Jeffrey,
.I only have the files that are  on the server. I think the source code is not there. It is a liferay commerce 2.05.
The external company create about 30 jar files, the war and  the lpkg files. That are the files they have worked.

I am new to java and Liferay but I has this task on my hands.
I suppose that I need the source code. I have the jar file. I do not know if that is enough.I am not trying to modify any module now.
Only  check if it is possible.