I am new to sharepoint!. Any help on this would be highly appreciated.I have extracted the wsp file by renaming it to cab. But when I open it as a website in Visual Studio, It does not have master file and CSS files. Can anyone please explain how to make changes to the wsp file using VS and to build and deploy it.
You cannot make changes to a already compiled WSP file. You need to have the Visual Studio Project instead. This Visual Studio Project packages all the assets to a WSP add the compiled dlls and every thing else.
I highly recommend tho read the basic information on the MSDN Create SharePoint Solutions first.