Definitely avoid those anti-religious anti-Judaism YIVO and their ilk and those forbidden movies. They will not only not teach you Yiddish, but rather they will indoctrinate you with their anti-religious ideals. And what they teach is a dead dialect that no real people even use in real conversations, as opposed to the living Yiddish lived by hundreds of thousands of real, living, Torah Jews throughout the world (North America, South America, Europe and of course Israel.)
The only way to learn Yiddish, is to live it. Live it with those living Yiddish, not those dying anti-religious breeds. Definitely spend time in a frum Jewish neighborhood where Yiddish is the first or second language of the frum Jews. And read Torah true Yiddish books. Frum Yiddish books written for children can be very helpful in learning the language. And send your children (when you have) to Yiddish speaking Yeshivos and Beis Yaakovs.