One of the best ways to improve your Spanish vocabulary when learning Spanish will be to read whatever you can get your hands on. Make it a mix of literature and news articles in Spanish. For literature, start with a book in which you’ll feel comfortable reading without having to look up every other word in the dictionary.
The ultimate question comes down to this: Is it better to read and try to guess at the meaning of words you are unfamiliar with in Spanish or to stop and look it up midstream?
The answer depends. There are benefits to both strategies. By reading fluidly in Spanish without stopping every other minute to look up a word, you not will save time and be able to read more, you won’t frustrate yourself when it takes you 20 minutes to read one page. This, however, could be a sign that you’re not ready for that reading material.
The benefits of actually stopping, looking up the word and then continuing is that you’ll feel better understanding what you’re reading.
In the end, you have to do what feels most comfortable to you and not frustrate you at the end. Learning Spanish should be fun and you should enjoy what you’re reading.
One thing you can do is look up only those words in Spanish that you think will significantly change the meaning of the sentence.
In the end, reading will help you tremendously improve your Spanish vocabulary and expose you to many Spanish idioms and Spanish idiomatic phrases.