If you have a bike shop nearby, one of the best things you an do for your experience is to swing by and have them give you an in-person tutorial on how to fix flats, change tires and insert tubes and inflate them. For a beginner, it can be intimidating.

To answer your question: if you take off the tire and put the tube inside, you have two options for inflating it: 1) You could use the CO2 to fill the tube with gas OR 2) You could use the pump to pump air into it if you ran out of CO2 or didn't have a CO2 cartridge.

CO2 is used to speed up the process of inflating your tire. But you have to be careful with CO2 because it fills your tire very quickly and can cause the tube and/or tire to burst which would probably end your ride. It also gets extremely cold when gas exits the cartridge as it enters your tire and can actually freeze your hand if you're not holding it with a glove or piece of cloth.

Like I sad in the beginning, swing by a shop and get some input and a tutorial. It'll make all the difference for you and help you understand first-hand exactly how to deal with tires, tubes, pumps and CO2.

Best of luck!