Connection b/w BW and External S/S

Hello ALL

If we need to extract data from external source system(say like Informatica,oracle,Java,etc.,)to BW system.Now what is the procedure/steps for connecting between them.(connections between Informatica,oracle,etc., & BW)

I came to know that by DB connectivity v can connect them.Is it true.

I need steps for this.Will the connections is to set by ourself or by basis people.

Pl anyone give me steps.

Regards

Balaji