If you are writing this, you would write 'fourteenth' as a number : September 14th. If you are speaking, you can say either 'September fourteenth' or September the fourteenth'. The first is mainly used in US English, the second in British English. You can also say 'the fourteenth of September'. This again is more common in British English.

It is also very common just to use numbers, ie. September 14th instead of writing out fourteenth. Unless this was for legal documentation or anything official.