Basically you should configure:
1. OPC Server - how to do it - nearest is doc: "How_to_set_up_OPC_with_Simatic_Net.doc". See this site Downloads/Siemens
2. OPC Client for using with VB. There are many of them in WEB. For Example - on the site of KepWare. You could download it with the code samples and Help.chm.
Regards, YuryAS.