One of the easiest ways to increase your overall fitness is by making yourself accustomed to a slight hunger for more air. There are many ways to do this. Common sense would tell you that if you wanted to feel like you needed to breathe you could just hold your breath. This can become stressful though and is not always easy to do periodically throughout your day.
A practice called Breath Light can be added it to many activities during your day though. While you are reading, watching TV, or working on a project you can practice breathing in this manner. The goal is to simply breath less than you normally would. There are a few different methods for creating an air hunger while breathing lightly. Personally I always appreciate a physical cue though. So instead of just abstractly trying to breathe less imagine breathing only enough air to fill your nose.
It will be tempting to speed up your rate of breathing as you do this. To keep your breathing consistent make your exhales slow. Breath in the amount of air that seems like it would only fill your nose then slowly let it out.
If needed you can take breaks to breathe normally every minute or so and reset your breathing. As you try this you will naturally find the best rate and amount of breath that makes the desire for more air mild but not extreme. If you are taking breaks and needing to breathe every couple of breaths adjust the practice so it is slightly easier.
This exercise is an easy way to build your CO2 tolerance and overall endurance. Add it in sporadically throughout your day and you will see your ability to breathe easier during workouts and play increase.