To Root For
To Cheer On, To Cheer For
する verb, transitive verb
応援 is aid and root for, but for this verb the "root for" one is going to be more on the money. You can call this to root for, to cheer on or to cheer for.
The reading comes from 応援. It's also just the on'yomi readings you learned.
どっちの力士を応援してるの？
Which sumo wrestler are you rooting for?
トーフグ社員はみんな、日本代表を応援しています。
All Tofugu employees are supporting team Japan.
成績が落ちたからと言って、好きなアイドルを応援することを我慢する気はサラサラありません。
I have no intention of refraining from supporting my favorite idol just because my grades have dropped.