Choose a chain ring before embarking on your ride. Twist the left grip-shifter to move the chain onto a different chain ring. Be sure to be pedaling forward when twisting, as the only way for the chain to move from one ring to another is when the pedals are moving forward. Use either your hand or feet to pedal while shifting. The left grip-shift controls the front derailleur. The front derailleur is the mechanism that moves the chain from one chain ring to another. Many bikes with multiple gears have three different chain rings. Twist away from you to choose a lower chain ring and toward you to choose a higher chain ring. Once you've chosen a chain ring, it's typical to leave your bike on that ring unless you are riding up extreme hills.
Start riding once you have chosen a chain ring. Shift up to a higher gear when it becomes to easy to pedal. Twist the right grip-shift away from you while pedaling.
Shift down to an easier gear when it becomes too difficult to pedal due to wind or when you're riding up a hill. You should also shift down before you stop. Shift into a gear that is easy to start up in. Twist the right grip-shift toward you while pedaling.
Continue to shift up and down, using the right grip-shift as needed on your ride. It's common to constantly shift up and down. There is no right or wrong way to shift, so feel free to shift as much as is necessary during your ride.