You first need to decide on how YOU want to send the control signals, once you have decided on a protocol you can then implement it in code.
Your PIC program needs to receive the data from the PC, either one byte or multiple bytes (depending on your protocol), then check what the data tells it to do, and perform that function.
For a very simple example, you could use single ASCII characters to set specific pins high or low - so, say 'A' means set pin a0 high, and 'a' means set it low - your program needs to take the received character from the USART and compare it to a series of valid characters, branching accordingly. If it receives a non-valid character, you could either discard it, or have some kind of error indication.