How can I sing without cracking?
It is always best to work with a teacher. You need to focus on having constant, balanced breath support and a refined, consistent phonation.
Meanwhile, a couple things you can play with:
Make sure your posture is tall and that your ribcage stays expanded the whole time you are singing.
Keep a little squeak at the center of the sound until you can do easy little sirens from the lowest sounds to the highest without a crack.
Then try to keep the breathing balance and phonation when you sing.