上る consists of a kanji with hiragana attached, so it will use the kun'yomi reading of the kanji. For 上, there are multiple kun'yomi readings, and you didn't learn this one yet, so here's a mnemonic to help you:
You want to climb the mountain, but a gruff old man grabs your shoulder and says "No boy, you ain't climbing that thing, not today." If you're not a boy, think about how weird it is that he called you a boy. If you are a grown man, think how you wish he called you "dude" or something, not "boy." If you're a boy, well... think about how appropriate it is that he said "no boy."
I can go up to the roof.
I climbed up stairs to the 10th floor.
I saw a salmon swimming upstream.