Ending with an い tells you that this word is an い-adjective. What's the adjective form of pleasure? Well, you could say it's "pleasurable," but I'm afraid that's not what we're going for here. In this case, this word is a little more innocent. It just means fun.
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:
How much fun did you have? You just had a ton o' (たの) fun!
Walking with a dog is fun.
It was fun seeing fireworks with my friends when I was a college student.
It's more fun to eat breakfast while watching TV than it is to eat while talking to someone.