What are some tips to be good at singing? I signed up for a play at my school, The Little Mermaid Jr, and I'm Ursula. My voice keeps cracking when I sing, and I am in desperate need of help.

Firstly try and relax as your chest will tighten and your voice will be strained.  Learn to breath deeply and same type of breathing when you run - inhale through the nose and as you exhale through your chest your sing and breath out and push out the breath and the voice.  Drink plenty of water;  Learn to do vocal warm ups before practising. At a pause in sentence of a lyric like a comma or full stop that is where you take a breath.  

Stop if starts to get sore or dry and take a break every 15 to 20 minutes.  Also you can try and use your Head voice which is your speech level voice and it might help when you go up on the higher notes.