This is the most obvious tip for improving your vocabulary, but it is the most essential. Reading various kinds of literature will not only expose you to new words, it will teach you these words in context.
This may be the most difficult step if you are not already in the habit of reading. It requires a bit of self-discipline, but it will be extremely beneficial. Set aside time each day to read. Try reading a variety of texts like newspapers, documentation, and of course fiction.
Another positive habit is daily writing. This is beneficial for numerous reasons, but it will provide an outlet for using new words to express your ideas. And most importantly, it allows you to intentionally explore new words on your own, without an audience.
Adopt this rule:
When you come across a word you don't fully understand, look it up and write it down.
You can keep a physical dictionary with you or make use of your smartphone. Either way, whenever you come across a word you don't know take the time to look it up.
Using Google search, you can type:
And look up just about any word. Google will quickly provide a definition and example usage.
If you're ambitious, consider learning Greek and Latin elements (prefixes, roots, and suffixes). This will not only help you learn new words, but it will help you know the meaning of new words you encounter. There are many free resources online to help.
Download Words With Friends. If you're frequently engaged in mindless activities on your phone, use that time instead to play a word game like Words With Friends. This challenges you to construct new words that you may have never used. As you play, look up words you don't yet know (just don't cheat).
Expanding your vocabulary takes time and dedication. This is not the kind of activity that can be achieved by binging. You must learn frequently and in small chunks. A few words a day is the maximum you should aim for.
As you're learning new words, use them. Don't be afraid to use them in conversation. You might even consider annoying your friends by over using new words. This can help you to remember.