Hey, you are the leader of the cheerleaders, right? I’ve heard a lot about you.
A group that roots for you is your rooting section or cheerleaders. Go go go you!
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well. 応援 is a word you've seen before, so if you know that this word will be easier.