Since this word is made up of a single kanji, it should use the kun'yomi reading. When learning the kanji, you didn't learn that reading, so here's a mnemonic to help you with this word:
You look outside, it's totally frozen. You throw salt (そと) outside to melt the ice.
Let’s wait outside.
The cats are staring at the bird outside of the window.
I kept yawning outside all day today.