Hi Marie, you don't want to hold your breath while swimming, except as an exercise or drill at practice. The proper way to breath while swimming a race is to time your breathing with your stroke, so you are breathing in every 3rd or 5th stroke, or whatever feels right, then exhale slowly while you are swimming with your face in the water. Then get to your inhale stroke, put your face up and breath in, repeat. When you hold your breath you are building up carbon dioxide in your body and it feels desperate for air, then your muscles tense a little and slow you down. If you want to get better at breath control and how long you can exhale between strokes then try breathing exercises like this one:

Take three very deep breaths, slow breath in then exhale, on the third breath in make sure you exhale all the air from your lungs completely. Then take a deep breath and hold it as long as you can. When you feel the need to exhale do it slowly and time how long you hold it then how long you can exhale it. You now have a benchmark to try and better each time you do this exercise.

But when you get in the pool practice timing and continuous exhaling until your inhale stroke.

Go get wet.