Mastering and growing vocabulary is the most important aspect of learning a new language, and it also can be very challenging. It takes time to actually retain a word, putting it in long-term memory. And the sheer number of words to learn over time can be overwhelming - for example, the English language contains more than a million total words, with about 170,000 words in current use, and on average 20,000-30,000 words used by each person.
The YouTube channel English with Ronnie offers great tips on how to remember words effectively, and this simple advice can be applied to any foreign language one studies.
Revise as much as you can
Repetition of new words is key. Every type of learner can choose a revision technique that suits her most: flashcards (for example, Quizlet or Anki), writing words down on sticky notes (it's also helpful to put them on top of corresponding objects), or visualizing words (for example, by drawing your own pictures). You can also make sentences with new vocabulary words and repeat them afterwards. And don't try to learn more than 10 words a day!
Another great trick to memorise a new word is to find a similar-sounding word in the languages you already know. It'll help you to remember a new word better even if there's only a slight resemblance between the two.
There's another similar and effective method: learning through word associations. It means associating a mental image with an unfamiliar word. For example, you can associate the word "ambitious" with someone like Elon Musk, or even come up with deeply personal and idiosyncratic associations that are only meaningful to you. You can then revise by matching associations with corresponding words and using them together in a sentence.