Learn Italian: Il nono mese è settembre. - The ninth month is September.

You are right that most adjectives come after the noun in Italian. However, ordinal numbers like nono (ninth), primo (first), and secondo (second) are exceptions! They almost always come before the noun they describe, just like they do in English.

Does nono change its ending depending on the noun?

Yes, it does. Ordinal numbers act like regular adjectives ending in -o. Since mese is a masculine singular noun, nono keeps its masculine singular -o ending. If you were talking about a feminine noun, like the ninth week (la nona settimana), it would change to end in -a.

I noticed settembre starts with a lowercase letter. Is that correct?

Yes! In Italian, the months of the year and days of the week are not capitalized unless they appear at the very beginning of a sentence.

Since mese ends in -e, how do I know if it is masculine or feminine?

Nouns ending in -e can be tricky because that ending does not give away the gender. You just have to memorize it! However, in this sentence, the words around it give you the answer: the masculine article il and the masculine ending on nono both show that mese is masculine.