Learning a language is not only beneficial for your career and it's advancement but also to work up the muscles of brain and to keep it working even when age turns against you. It also opens up the opportunity for one to explore new cultures and adapt good things from their tradition. Here are some tips that you can follow to learn a language effeciently and quickly.
When learning new vocabulary, write the meaning of that word above it, that way whenever you revise that word, you can refer to its meaning immediately.
If you are learning a new writing style (i.e. kanji, urdu etc.) write the character as many times as you can to get used to its strokes, the meaning and the pronunciation with it. Do not write the pronunciation of the word/character above it, write it on the side, or else you won't even attempt to read it. Also pay attention to the stroke order.
Do not pay attention to the technicalities of the grammar. Do not try to find commonalities of it with your own language, that will just confuse you. Learn the rules of sentence structure and practice making sentences in your target language.
If you are learning a tonal language (i.e Chinese) or language that has sounds that don’t exist in your language, I recommend watching videos of pronunciation on YouTube and try to match your mouth movements.
Watch TV shows in your target language with subtitles. But please be aware that may not be how people speak in real life (such as Japanese/Chinese/Korean). You can learn the pronunciation, the correct way of exclamations and such but take care that you don't overuse the slangs used in the shows.
Do not be afraid to make mistakes and fumble. If you mess up during a sentence just correct yourself and keep going.
Spend at least an hour a day on it (other than class). If you’re learning on your own, you may need more time ( and discipline). Be sure to give yourself enough time to grow your concepts.
Talk to yourself in that language, think in your target language, make your lists and notes in it, set your phone to it. You probably look crazy but that is okay. What matters is your aim.
Listen to music in that language, while it probably won’t do much for your ability in the beginning it will help you distinguish sounds once you get pretty good. Also, you can watch movies in your target language. They don't have to be made in it, you can watch the dubbed version. Especially the dubbed version of animated movies, such as Disney and Pixar. As they are made for children, the language is fairly easy and the accent easy to grasp. You could watch the classics in that language, too. Read books for children in that language. They are fun to read and for the beginners, an easy way to build up on vocabulary and improving sentence making.
And lastly, don’t give up. It took you a decade to know your language properly, it's gonna take a while to learn another. Consistency and perseverance is the key.