What's a good entry point for The Idolm@ster?
How about this youtube channele?
You can watch promotional videos and radio programs that you can watch for free; original, Million, Cinderella, Shiny and SideM are all treated.
Anime and specials are distributed depending on the time.
Since these are Youtube, most content has auto-generated subtitles;　
somewhat inaccurate, but automatic machine translation is possible.
(If you choose English or ’英語’.)

Of course, these are legal content operated by copyright holders.