When I first started doing vocal exercises, I wanted to sound like Bruno Mars and David Phelps (I STILL don't) but then I realized that I needed to sound like David R. Henry. God made me special. I am tall and formerly thin and have a high baritone voice. Frank Sinatra and Elvis suit my voice perfectly.
When I tried to do Bruno Mars or Michael Jackson, I squeaked and sometimes COULDN'T even hit the high notes. My vocal cords wouldn't close and let me hit those notes. I had to belt to reach my upper head voice. It even gave me a headache to sing up there.
Voice crack is a sign of too much tension and pressure in your throat. It's OKAY to crack and sputter when you are practicing. Your vocal cords are learning a new coordination. EMBRACE those ugly sounds. They won't be around for very long.
One trick you can do is to reduce the volume of the sound as you go higher. Avoid the temptation to push and belt for the high notes. You'll be amazed at what reducing your volume at the top will do for your control.