Sometimes, it is appropriate to use numerals in dates, people's ages, or percentages. At other times, however, it is more appropriate to write out the number as a word, such as at the beginnings of sentences. If your teacher wants you to use a specific style guide such as the MLA or the APA style guide, then consult those style guides for specific instances of how to write numbers in words, as they may differ.
Rules for Writing Out Numbers
The general rule of thumb is that if the number is one or two words, you should write it out as words, rather than using the numerals. This means that you should write out numbers zero through one hundred. Remember to use a hyphen for numbers twenty-one through ninety-nine. You would also write out numbers such as one thousand, three million and numbers such as two billion, but these numbers don't need a hyphen. You can show numbers that require three or more words to write out as numerals, because it would be cumbersome to write and read these numbers. Therefore, you would write 1,001, 235 and 5,783 as numerals, not as words.