I did History for my BA, and currently teach undergraduates, and don't worry, this is a very common reaction from history students!
I would begin with the essential texts - these are likely quite broad books that give you an overview of a time period or topic, and will help you get a grasp on the chronology, the chief characters/events, and the major debates within the subject, so it's worth reading a couple of these. My gut feeling with regards to the 'reference' texts with history is that they might be useful for looking up specific people or events to help you get a further grasp on particular things. You can also use online resources such as the Oxford Dictionary of National Biography to help with this - it's a database of major figures in British history, and gives a potted biography for each person.
You will find that often the subsections within the list will follow the lectures as given by the lecturer. One way of trying to figure out what is worth reading is taking a look at the footnotes or references of one the essential texts, and see what they have relied upon when writing about that particular topic. If you have to read for a seminar with a specific topic, look for journal articles that might help you get to the point much quicker than reading a book; as claireestelle said, looking at the index and contents page of books can also help you get an idea of what the book contains, and you may only have to read 1-2 chapters from that book. For journal articles, learn how to search JSTOR and Google Scholar effectively, and use databases such as the Bibliography of British & Irish History (for things dealing with British & Irish history, obviously
) to find any pertinent articles.
Above all, don't panic! You are not expected to read absolutely everything on the list! Part of the challenge is meant to be learning how to choose what to read, how to find further things to read, and most all learning to read critically. If you've any further queries I'd be happy to help