2000 Chevy Malibu check engine light

I have a 2000 Chevy Malibu with a 3.1L V6 with 139, 8xx miles on it. The check engine light stays on while the car is idling but flashes when the car is moving at about 35 mph. I went to autozone to get the OBDII code read, and I got the P0171 code (fuel system for bank 1 was too lean). It shows multiple causes (O2 sensor defective, mass air flow sensor, or fuel injector problem). How do I know which problem to fix or which O2 sensor to replace? I'm in the military and can't afford to replace every part until I find the right problem. Thank you in advance for your services.