Need help tweeking gears please - Cycling

What's going wrong with the gears? Is it a new bike? I'll take a couple shots in the dark: Both the front and back derailers have adjustment screws (usually two). These are only limiter screws and they set how far out and in the derailer can travel. If your chain is slipping off either the biggest or the smallest gear then you may need to adjust these. More often, especially on a new bike, the cable worked initially but has since stretched slightly. This will feel more like many, if not all, of the gears want to slip to the next gear when you put power to the pedal. To fix this, look at either end of the cable housing (plastic part) for a knurled adjustment. Screw this "out" so that it extends the housing. Often this adjustment is up near the shifter lever/knob/etc and can be adjusted as you ride (of course watch for traffic). Fine tune this knob till the gears shift easily and stay when peddling hard. Try screwing one way, then the other till things improve. Good luck!