A few good ways to learn new words are:
1) Use a monolingual dictionary to look up words you don't know. Say you don't know the word "cat." If you look it up, you get Cat (n): a carnivorous mammal (Felis catus) long domesticated as a pet and for catching rats and mice. Now you've read the definition and understood the words you know, but say you don't know "carnivorous" and "domesticated". So you look them up and learn two more words, plus the words in their definitions! This is my favorite way to get vocabulary.
2) Read books in your target language! Find an English copy of your favorite book and read it. It'll be slow and frustrating, but if you really like the story, it will be worth it!
3) I really like the website Memrise to learn vocabulary.
I hope one of these works for you! Good luck :D