I am installing a PVC pipe to a sink. The joint between the sink, and PVC pipe has a small leak. How can I seal the leak?

Are you using correct pipe for this type of installation ? You should use "tail piece" pipe with flat washer. That washer fits into the flat top of this pipe. Also, use metal nut to attach tail piece to the sink strainer.

Yes. The PVC pipe has a dish washer connection. The pipe has a PVC nut. Do I need to install the dishwasher between the PVC pipe and sink?

Make sure that DW hose is not pulling the drain pipe sideways.

I am enclosing photo what it should look like. Depending on your situation, you will use one of these two Branch T tubular drain pieces...

It would be great if you could snap a clear picture of it and post it...