The average person learns most of the 30 000~40 000 words whose meanings he or she recognizes by hearing them or getting familiar with them in the context or simply absorbing them without conscious effort. The best way to build a good vocabulary, therefore, is to read a great deal and to participate in a lot of good talks. There are relatively few words that we learn permanently by purposefully referring to dictionaries or keeping word lists. However, even those extra few are of value, and no one will make a mistake by working on developing a larger vocabulary. Here are some suggestions of how to do it. Read plenty of good books. When you come across a new word, or a new meaning of an old word, stop and see if you can understand it from its context. If you can't, and if you can manage without interrupting the thought of the book too much, look it up in a dictionary or ask somebody and then repeat its meaning to yourself a couple of times. If you are really conscientious , write the word and its meaning in a personal vocabulary list - preferably using it in a sentence, or you can keep a special vocabulary notebook. Go over the list from time to time. Further, try to use a new word in writing or conversation a few times over the next several days. Listen to good talks and be alert to new words you hear or to new meanings of words you already know. Then treat them just as you treat the new words you read. Learn and be alert to the parts of words: prefixes, suffixes and roots. Knowing _ enables you to make intelligent guesses about the meanings of words. If you are studying a foreign language, be alert to words in that language which relate to words in English. English has inherited  or borrowed much of its vocabulary of 500 000~600 000 words from Latin, Greek, French, Spanish and German. When you meet a new word in reading, what should you do?
Guess its meaning. Ask somebody. Look it up in a dictionary. All of the above.