You can always use Spark Notes or other such sites. The actual text which you will buy has fairly interesting and in depth character, plot and motif summaries at the back as well as small anecdotes on the left hand side of each page that helps to gain an understanding of the language and play.
York Notes are the best but they are $15. My recommendation would be to go through the actual text and the notes within it which will give you both a textbook reading whilst also allowing you to come up with your own interpretations and ideas.
Also, look at the historical background in which the play is set....for example, Queen Elizabeth is barron, what the world and society was like and try to make links with characters such as Hal and the juxtaposition of the Court/royalty with the taverners. Think about what Shakespeare is trying to say. It might not be correct, but if you can argue in such a manner, this is the higher level thinking that will get you 9s and 10s.