First post, need help!

Hey,
My girl owns a 2002 Cav ls1. Her gas mileage has fallen significantly in the past tank and a half, as in, she's almost to half a tank and only at 110 miles.
I want to replace her fuel filter an PCV valve, but have little knowledge of the Cavs.
It appears that there are no Haynes or Chilton manuals (please correct me if I'm wrong) and I was wondering if anyone here could help me out by suggesting places to buy these parts and giving me a rundown on how to replace them. (let me know if a midly mechanically inclined person can do it without special tools, also please.)
Thanks in advance!!

warranty work! The first thing is to trake it to a GM dealer because even the most basic warranty should apply to the car depending on the mileage.

the fuel filter shouldnt decrease gas mileage enough to notice check the spark plugs. if the fuel filter is clogged that will just decrease power. also check the wires.