Long
い adjective
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 long? It's also long.
ながい
Since this word consists of a kanji with hiragana attached, you can bet that it will use the kun'yomi reading. You didn't learn that reading with this kanji, so here's a mnemonic to help you:
You have something that's really long. It's this crazy hair growing out of the tip of your nose. You don't really care either way if it stays or goes, but you're too lazy to cut it off. Your parents, though, keep trying to nag (なが) you to trim it. Imagine this process happening and be irritated about it. What naggers.
このえいがは長かったです。
This movie was long.
あそこのドアの近くに立ってるかみの長い女性が、たん当しゃの方です。
That lady with long hair standing by the door is the one in charge.
長い距離のランニングを避けるため、自分の足の骨を折ってしまおうと思っている。
I'm thinking about breaking my own legs to avoid going long distance running.