It depends on what the numbers are referring to. For ages, prices and dates, you usually just write the digits. For quantities, it can really be either, but writing the word is slightly more formal. At school, I was taught to write out the word for smaller numbers under 20, and to use digits for larger numbers (so you might say "He has ten apples" or "He has 500 apples". But to be honest, it doesn't really matter except in formal writing.