Onyomi is typically what is used in words with more than one kanji. I'm going to give an example of some kanji with multiple readings. Onyomi usually aren't used by themselves, except for some exceptions. Kunyomi can by used by itself. It's usually safe to assume words made up of more than one kanji are read with onyomi.
観 Look カン み（る） To watch(movie, play, concerts, things you watch for a long time)
光 Light コウ ひか（る）To shine to be bright、ひかりlight(noun)
観光 Sightseeing(what tourists do.) Read as: かんこう
光 is read as ひかり. 光る is read as: ひかる. 観る is read as みる.
One more example.
教 Education キョウ おし(える) To teach
室 Room シツ
教室 Classroom(literally "learning room.") Read as: きょうしつ
教える is read as: おしえる
One important thing you might of noticed, verbs(i-adjectives too) have extra kana hanging off the end of the kanji, this is called "okurigana." There are kanji with more than one kunyomi, the okurigana tells you which kunyomi reading it is. That is how you can tell the difference between 光 and 光る when reading. The verb has okurigana, and the noun doesn't.