To decide what to learn, use the 80/20 principle. This means I learn the ~500 most used words in the target language.
To study vocab, don't memorize translations! Instead, create a vivid mnemonic story around each word, encoding its pronunciation, its spelling and its meaning. Make the story personal!
Then, use flashcards with pictures, not translations! Review at the optimal time with spaced repetition. On every review, visualize the story you came up with.
Next, learn grammar [...] gap sentences (answer: with). You can use the same sentence with different gaps to [...] all parts of speech (answer: practice).
Once you have the basics, the key to speaking fluently is mirroring. Find a person you like who is fluent in your target language. I used a uni friend for German, GF's for Portuguese & Chinese, colleague for French. A famous actor or cartoon character works too!
Pay close attention to how they speak, and try to mirror their way of speaking whenever you use your target language.