It's easy to build 'essential' vocabulary with In Use series - vocabulary in use, idioms in use, phrasal verbs in use.

The more you read the more vocabularies you'll get.

Learn vocabulary in the relevant context. Repeat the vocabulary the next time you are in that situation. It will stick naturally, without trying to memorise.

make word-lists

This is the first time I'm asnwering a question here :) From my experience I can tell you: 1. Never try to memorize individual words on a list. You'll forget almost all of them anyway. 2. Never ask your teacher to teach you new words. They don't care about you and it isn't effective. The best way to learn new words is by hearing/seeing them in real/authentic material such as podasts/books. Choose a podcast that you're crazy about and listen to it repeatedly every day. This way you'll internalize new words and, most likely, never forget.