Nagoya
proper noun
The place that's famous for old roofs. I don't know why, but it's the city of Nagoya. Just associate the two things together and it'll be hard to forget (so long as you've heard of Nagoya before - if not, read up about it!).
なごや
This is a jukugo word, which usually means on'yomi readings from the kanji (except for 名 and 屋, which are kun'yomi here). If you know the readings of your kanji you'll know how to read this as well. For な just think to 名前 (なまえ) which uses the な reading. The "name" (名前) of this city is what? It's Nagoya. Watch out for rendaku changing こ to ご too.
私の友人は名古屋でデザイナーをしています。
My friend is a designer in Nagoya.
名古屋の朝ごはん文化は全国で有名です。
The breakfast culture in Nagoya is famous all over the country.
名古屋市内一、高いビルはどれですか？
Which is the highest building in Nagoya City?