Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You haven't learned this reading yet, so here's a mnemonic to help you:
If you want to get thin, just join the Yankees (や). They run their players around those four bases so much they all get thin quicker than you can throw a ball.
It's been a couple years since I started saying, "I want to lose weight."
Most stray dogs in my village are skinny.
When it gets silent and awkward during conversations, I make it a rule to just say, "Have you lost weight lately?"