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:
This year you plan on tossing (とし) out all your old stuff as a New Year's resolution. You pronounce it "toshing" though, because it has more vigor that way.
I hope you have a happy New Year!
Once again, I was able to age this year without issue.
The next year, Erika met a prince riding a white cow.