Hi Denise,
Atholl is quite correct. While there are guidelines and conventions about using words or numerals, this usually depends on context. For example, a technical manual or scientific treatise may prefer numerals all the way through for greater clarity, even for amounts less than 10, while books (fiction and non-fiction0 usually follow the idea of whole numbers up to ninety-nine are words while three, and four-figure numbers and above tend to be numerals, apart rom round numbers.
Newspapers and magazines, however, may well use numerals for 10 and above. In any case, you should aim for consistency but chapter seven of Fit to Print gives a good overview.