The beginning sound of "cat" is /k/ because the letter "C" in this word makes the hard "k" sound. It's a consonant sound made by briefly stopping airflow in the back of the mouth and then releasing it.
Here are some similar examples:
- "cat" – /k/
- "cup" – /k/
- "cot" – /k/
- "car" – /k/
- "cake" – /k/