If 21% of our atmosphere is oxygen and 78% of our atmosphere is nitrogen, are we able to attain our nitrogen levels in our body that we need through the air?

We breathe in everything, whether it's good, bad or useless. If it's a gas or suspended particulate in front of your face and you inhale, you're getting a lungful of it (or at least throatful in the case of particulates). Nitrogen, in the form of N2 (which is the gas present in the atmosphere), is incredibly inert. That is to say, it's stable- it doesn't really want to react with anything anymore. When you breathe in N2, you breathe out N2. It hasn't changed and hasn't been absorbed by your body (at least in any significant way). Thus, it is neither good nor bad for you- it's useless (nitrogen content of air becomes very important when diving at great depths, but for the general purpose of this question it's not important). Oxygen, on the other hand, is in the good category- it is absorbed by the lungs and taken into the body where it is converted to CO2 in a process described in greater detail by other authors.

That's why people say that we breathe oxygen- because while we inhale everything, we only use the oxygen.