How do I stop water from seeping between my porch roof and house siding?
I'm going to bet you need kick-out (rain diverter) flashing to divert the water off the roof and into the gutter at the point where the roof/wall intersection ends. It is probably the most left-out piece of flashing and causes lots of problems.

 -kick-out flashing



However, there is no way to tell where the leak is coming from without a thorough inspection of the area. First step would be to check the roof/wall intersection above for proper flashing.