This word consists of kanji with hiragana attached. Because the hiragana ends with an う sound, you know this word is a verb. The kanji itself means count, so obviously the verb version is to count.

Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:

You begin to count vocabulary words. You're out of control and you can't stop. One after another. Scientists have to come in, but they can't figure out the cause (かぞ) of your counting. They drag you away to a padded cell where they keep testing you trying to figure out the cause of your craziness.