Help with Japanese kanji?

I know that there are typically two reading for Japanese kanji: On and Kun. Like the kanji for hana (flower) the On reading is Ka and the Kun reading is Hana. For most of the kanji I know, you usually use the Kun reading. But what I don't understand is can you use both readings for a word? For example, since the On reading for hana is Ka, could you use Ka instead of hana? Like if I was reading something in Japanese and I came across the kanji for flower, could I pronounce it as Ka? Can someone please explain Japanese kanji for me?