Read everything that you find, from newspapers to novels to blogs. When you read, you'll not only come across words you don't know, but you'll develop a feel for the flow of language and the appropriate usage of grammar. When you read good writing, your own writing will naturally become more fluid.
Keep a log of words that you come across that you don't know. You may keep a small notebook in your pocket or jot notes on an application on your phone. At the end of the day or the week, look up the words and put them in a place to review later. Make it a goal to use a few of these words in a sentence the next day.
Sign up for a free word-of-the-day newsletter from a website, such as Dictionary, WordSmith or Merriam-Webster. Each day you will receive an email with a word, its definition and its use within a sentence. As with your vocabulary notebook, put this word somewhere to review later and think of ways you may incorporate it into your writing or conversations.
Give yourself 15 minutes to write freely about anything you like. Jot down what's happened in your day. Also make a list of things you're grateful for; brainstorm ideas for a novel. When you write freely rather than writing for a purpose, you will discover surprisingly good writing within your thoughts.