What is the best way to learn a language? I have to learn Spanish and they suggested me Rosetta Stone? Does it work? Are there better ways?

Mother tongue tbh. Children are sponges. They absorb information so rapidly. Kids should be taught English, Spanish, Mandarin, Italian, French, Japanese, etc. from birth. Then they'll just grow up knowing multiple languages as easily as we all know our native languages.

The title confuses me. I wanna learn Japanese tho ! I started by yeah, am not the most motivated person. I used the sites Monte gave me ;_;

I studied Japanese at uni and it was pretty good - unless you're extremely motivated, I think the financial obligation to having to learn and pass a class is worth it. Plus moving to a country and immersing yourself.

今年あまり日本語練習しなかったから、日本語を忘れていました。

True, but not gonna help the prophet MaRy now is it?

Try Duolingo.

This doesn't answer his question

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Try duolingo, it's free and effective. I would recommend mixing yourself into groups of people that speak that x language, that's the best way.

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best tool? not sure.

Classes to a proficient level then immersion.

This. You'll never get fluent without literally a year + of daily use.

Duolingo seems good. Classes are optimal. Immersion is the goal

private tutor

College or surround yourself with people who speak that language

I've been learning Mandarin in the university. I think it is a good way and cost efficient. I am not so good at it but if you show some motivation you can get really good.

Immersion.

Nothing beats classes. Look for a night course at a local community college, I bet they are affordable. Once you feel comfortable enough in the language then you should try immersion, if you live in North America there is such a big Spanish speaking community you don't even have to travel to South America or Spain to practice Spanish. Every large Canadian and American city has a community of Spanish speakers.

I wouldn't suggest immersion without taking introductory courses first.

Classes then immersion. Anyone who thinks they can teach themselves a language using a smartphone app is a fool.

Apps can help after the basic classes.

How to learn:
1. Take a class for the sole purposes of learning basics: Alphabet, grammar and pronunciation
2. Immersion/focus on something you enjoy and do frequently-- i.e gossip or music or cook w/e
3. Expand vocabulary
4. Visit country (ies)
5. You're welcome

I use Duolingo but a class is the best option

I meant "solely" using an app, which a lot of people believe is possible. It's not impossible, but near enough.