Numeral
ななつ
はこのなかにりんごが七つあります。
There are seven apples in a box.
二人でメロンを七つたべました。
Just the two of us ate seven melons.
七つかぞえると、わたしは女になる。
After I count to seven, I will be a woman.
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 seven plus つ. So, this one is seven things.
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).
What thing do you have seven of? The seven things that you have are seven Nanas (なな). These are your grandmas, cloned. Just think, "It's lucky to have seven nanas. They can all cook my favorite foods for me!"