The formal classes (such as YIVO’s) are of limited value since the course materials were designed by secular Jews and reflect the Yiddish spoken among the frei Jews 100 years ago. It is for all purposes a dead dialect. Sometimes the changes are comical (if you remember Sen. D’Amato, he used a Yiddish phrase he had picked up to try to win frum votes – the phrase was a reasonable insult from the Lower East Side, but a gross obscenity in frum Yiddish – which is how be became a lobbyist instead of a senator).
If you know Hebrew, you already know at least a third of the Yiddish word stock. If you know German, you probably know another 50% (English, is related to Germany, so knowing English gives you 10% of the vocabulary. Modern (meaning frum) Yiddish is a lot more like English than it was 150 years ago in terms of grammar.
There are a lot of children’s books produced by the frum Yiddish communities that reflect the 21st century Yiddish that is still a living language, and using them to learn the language should be fun. Whle there isn’t much online, there are analog audio materials (CDs) produced by and for the frum Yiddish speakers, or you can visit a place where Yiddish is spoken (places in Brooklyn, Antwerp, places in Eretz Yisrael).