Depends on your definition of 'fluent'. To speak like a native, you probably need to spend several years in a community where your target language is spoken as primary language. However, you don't need to be fully fluent to passably talk, read and write even on advanced level (like politics or academic subjects).
Find the part of the internet that communicates in your target language. Read and participate. Structured language classes with a professional teacher are helpful, but not sufficient.
Duolingo is nice for very basic introduction and first "stepping stone", but not enough if you want to progress farther than the basic tourist level (at least, not in the languages I have tried it).
After ~15 years studying (both in school and other activities), I wouldn't consider myself fully fluent in English but capable enough to get by in professional context.
Find the part of the internet that communicates in your target language. Read and participate. Structured language classes with a professional teacher are helpful, but not sufficient.
Duolingo is nice for very basic introduction and first "stepping stone", but not enough if you want to progress farther than the basic tourist level (at least, not in the languages I have tried it).
After ~15 years studying (both in school and other activities), I wouldn't consider myself fully fluent in English but capable enough to get by in professional context.