The reason people get confused about how to write numbers is that different style guides make different recommendations, so it’s easy to hear different rules from different people.
In general, the Chicago Manual of Style wants you to write out the words for more numbers than other style guides. For example, the Associated Press and the British newspaper the Guardian both recommend that their writers use the words for numbers less than 10 and the numeral for anything bigger: Today, firefighters rescued nine cats from trees. Yesterday, firefighters rescued 11 cats from trees.
But the Chicago Manual of Style recommends recommends using the words for all whole numbers 100 and lower, and also for big round numbers such as one thousand and twenty thousand: Yesterday, local firefighters rescued eleven cats from trees. Last year in the US, firefighters rescued 728 cats from trees.
There are a huge number of exceptions to these rules. If questions about numbers come up a lot for you, you really need to get a style guide and look it up. The Chicago Manual of Style has a whole chapter just on numbers.
These and more useful grammar tips can be found on http://grammar.quickanddirtytips.com/