Two Things
numeral
This word follows the "number of things" pattern where there's a kanji for a number plus つ on the end. Whenever you see this, you know the word means "____ things." Knowing that, as long as you know the kanji (which you do) you can figure out what number of things it is. For this one, it's the kanji for two plus つ. So, this one is two things.
ふたつ
Since all of the "number of things" words all follow the same pattern (number plus つ), you really just have to remember the part before the つ (and then remember that the つ is in all of these). All of the readings are the kun'yomi reading, which means we'll have to use a mnemonic to remember them (you know the on'yomi reading of the kanji portion).
So what thing do you have two of? The two things that you have consist of one foot and another foot (ふた). Remember from the on'yomi how you had two knees (for the reading に)? Now look further down your leg at one foot then over to the other foot. Two foot!
二つのかいわです。
It’s two dialogues.
このカゴにリンゴを二つ入れてください。
Please put two apples in this basket.
かれの二つめのあくぎょうは、かえるのあんさつだった。
His second evil deed was the frog’s assassination.