SFDC Integration with SAP PI

Hi Experts,

I am working on SFDC integration with SAP PO AEX.

I have gone through various posts/discussion on salesforce integration but I have still one open query and so writing this.

1. Is it always mandatory to get session ID from salesforce to proceed ahead with various calls?

2. Do we need to specify any user and password to get the session ID or we can send any constant?

3. I have seen guys using a JAVA mapping for getting the session ID in the SOAP Header. Is there any alternative as I am poor in java.

4. In SHort if anyone can provide me step by step procedure for an interface I will be really obliged as I have less time.

Thanks a ton

Anand.