What's a good language learning app or method?

I'd like to learn Mandarin and also have interest learning Russian, Arabic and Spanish, but don't really know where to start. I know apps like Duolingo and Mango exist, but are those the best options? Would I be best off taking a class? To my multi-lingual comrades, where should I start?

Edit: Thanks for all the great replies everyone! I have a lot of resources to look into now.