Firstly, your lungs have limited capacity. Unless you let the air go out of your lungs, you won't be able to inhale.
Inhale and hold your breath. Now try to take another inhale on top of your previous one. You can't, right?
You have to exhale first.
Learn to exhale by blowing the bubbles in the water. You can blow the bubbles with your mouth. In this case, leave a tiny gap between your lips and push the air out of your mouth as if you are blowing soap bubbles or trying to whistle. Try to hum to let the air out to blow the bubbles with your nose. Pretend to sing a song with your mouth closed, see how you hum, and push the air out of your nose.