The sun radical and sun kanji are the same, huzzah! Also, if you want to remember day as well, you can just think about how a day is the time it takes for the sun to cross the sky.
ひ, か, び
There are so many on'yomi for this kanji, but since this kanji is so important and common, you'll end up learning them all eventually whether you want to or not. For now, you're only going to have to learn one of the readings, and that one is にち.
The sun is so hot. It focused its rays to take out one man... and that man is Friedrich Nietzsche (にち), famous philosopher. He philosophized one too many things about the sun, so the sun decided it was time he paid.