You will usually encounter new words when you read. If there are too many new words for you, then the level is too high and you should read something simpler. But if there are, say, a maximum of five new words per page, you will learn this vocabulary easily. You may not even need to use a dictionary because you can guess the meaning from the rest of the text (from the context). Not only do you learn new words, but you see them being used naturally.