While using a new app, I found this very helpful tip.
Almost every character has two different readings called 音読み （おんよみ） and 訓読み（くんよみ）. 音読み is the original Chinese reading while 訓読み is the Japanese reading. Kanji that appear in a compound or 熟語 is usually read with 音読み while one Kanji by itself is usually read with 訓読み. For example, 「力」（ちから） is read with the 訓読み while the same character in a compound word such as 「能力」 is read with the 音読み （which is 「りょく」 in this case）.
No one ever mentioned this to me in my almost two years of studying Japanese… The app is called Tae Kim’s Learning Japanese and it can found in the iTunes Store.