Alice:

I will bellow in a loud voice on the bus
"That is thief"
because I want to know everybody on the bus

Correct:
I will bellow in a loud voice on the bus.
There's a thief!
Because I want  everybody  to  know   on the bus.

Hello Alice! Thank you for submitting your homework on Talk Fun website. You have a clear opinion regarding the situation from the lesson. You were able to compose your idea. I encourage you to please observe the punctuation marks to complete the sentences. Besides, the determiner "that" is used to point near objects, while there is used to show something exists or happens. Just add verbs to be "is" since it's singular to modify the noun thief. I look forward for your homework again, next time. See you in class! Thank you.