One great way is to read, whether it is a book, a newspaper article or information on the web. Keep a notebook with you and write down any words that you don't know. Then, look them up in a dictionary. These days there are many great online dictionaries, with www.dictionary.com being one of the best that I've found.
Once you write down the words you want to learn and look up them up, also take a look at synonyms for them. Now you not only have one new word to use, but several to choose from!
For example, let's say you see the word "superfluous" in an article somewhere and you don't know this word. On www.dictionary.com, this word is defined as follows:
Superfluous is an adjective that means being more than is sufficient or required; excessive
unnecessary or needless
obsolete-possessing or spending more than enough or necessary; extravagant.
The above definitions for superfluous also included some synonyms. There you go! If you want to check to see synonyms for this word, click on the "thesaurus.com" on the dictionary.com website, which will take you to another area. Insert the word "superfluous" to view a plethora of synonyms to use. Now you have some great alternatives in addition to superfluous to use!