Read as much as possible. If you come across a word you don't know, write it down or look it up.
Use a dictionary. Many browsers include dictionaries. If you are reading online, you may be able to right-clcik on a word to look it up.
Sign up for a 'word a day' email.
Think of ways of remembering words. Do they sound like a word in your own language?
Remember words are often used together with other words. Try to learn these 'collocations'.