VISA is an API that allows you to talk to instruments. It provides a way to interface to GPIB/Serial/TCP-IP without having to write lots of duplicate code. There is a brief
KB article on serial instrument control that would apply to you. There's also examples that ship with LabVIEW. Open the Example Finder (Help -> Find Examples and expand the "Hardware Input and Output -> Serial" tree node. The "Basic Serial Write and Read" is a simple example you can look at.
You will need to adapt it to whatever protocol you're using with the PIC. For instance, what is the PIC using to indicate an end of a message. What does the PIC need to indicate the end of a command?