This is a single kanji with an い on the end, meaning you know it's probably an adjective. What's the adjective form of cold? It's cold.

寒い is used specifically for cold weather or when you feel cold. You can say it when you're outside on a chilly winter day, for example, or when you walk into a cold room. It can even be used as a response to unfunny jokes that leave you "cold."

Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You learned it with the kanji, but here's a mnemonic just in case:

Picture a cold samurai (さむ). Imagine him shivering outside on a winter night, with his katana rattling in its sheath. Icicles in his beard, frost on his topknot. This poor, cold samurai.