For english speakers, korean is the easiest east asian language to pick up as their linguistic sounds overlap the most and the korean alphabet is modular/phonetic. Meaning different characters have specific sounds, rather than characters making up entire words or ideas.
Japanese is supposed to be one of the harder languages to become fluent in as there is basically 3 scripts used in everyday life, hiragana, katakana and Kanji. not to mention the occasional "romanji." In contrast, mandarin just has kanji and modern korean is almost 100% hanjul. Though, mandarin is a tonal language and many words will have different meanings depending on the inflection and the context. It can be a little daunting even for koreans and japanese even though chinese is sort of a "root" language for both, since those two languages are not tonal.
Anyways, the rosetta stone program, I found, is a pretty effective system for learning a foreign language. It's pretty straightforward and it teaches you to think in the language rather than doing a lot of memorizing and translating.