Final answer:
The word in the sentence that should begin with a capital letter is 'My'.
Step-by-step explanation:
In the sentence, "My mom said, 'to get respect, you should first earn it.'", the word that should begin with a capital letter is 'My'. The word 'My' is the first word in a sentence and all sentences in English begin with a capital letter.
Learn more about Capitalization in English