The character 古 means "old". Its most common Japanese reading, or kun-yomi, is ふるい ( furui) ; in kanji, with the grammatical ending, it is rendered 古い. The sentence 私の車は古い (My car is old) employs this reading. In compounds with other kanji the character usually adopts the Chinese reading, or on-yomi. The latter is "ko", or こ in hiragana, and consequently the reading for the compounds of which it appears is most often "ko". Some common compounds pronounced "ko"are:
固- hard, firm
故- old, reason
Knowing that the pronunciation of such compounds is "ko" will help you to search for their meaning in Japanese or romanized dictionaries.
When the character retains its Japanese reading in compounds, the "i" is dropped, as in 古里, "furusato" (hometown, historic village); 古紙 "furugami" (old paper); and one of my favorites, 古臭い "furukusa(i)", literally "old and smelly" but meaning "old-fashioned; hackneyed, trite".