Read books, magazines and articles on as many topics as you can. By reading about topics that interest you as well as branching out into subjects you are not currently familiar with, you will come across new words that you may not have seen before. This makes the learning process much more entertaining and will often give away the meaning of a word just by seeing how it is used in the context of a sentence or paragraph. The more you read, the more your vocabulary will increase day by day.
Write down any words you do not understand in a notebook and set it aside.
Look up the meaning of the words you wrote down by searching for them in a dictionary.
Write a couple of sentences for every new word you have learned. By using the words yourself in new sentences that you create, you will further reinforce the meaning of the words in your mind. This makes them easier to remember over time.
Check your work over with a teacher, parent or an expert in the field of English to see if you used the words properly in your sentences. This will allow you to correct any misunderstandings about the meaning of a word.
Study all your new words consistently and start using them in your day-to-day conversations. The more you use new vocabulary that you have learned, the better your command of English will be overall.