I'll weigh in just to offer a counterpoint. I mentioned in a comment above why I find starting from secondary sources unsatisfying. They are almost uniformly skewed if not simply wrong, and you have no way of sorting the good ones from the bad ones or understanding what type of intellectual position-taking is motivating a reading if you haven't read the thinker himself. If you'd like to read Being and Time, you should read Being and Time, and then if you're unsatisfied read more about it, and then possibly go back and re-read it.
It might be helpful to start with an essay by Heidegger if you'd like something which is a little easier to tackle. The "Letter on Humanism" in Pathmarks or "Anaximander's Fragment" or "Nietzsche's Word: God is Dead" in Off the Beaten Track are relatively approachable and deal with fundamental aspects of his thought.
As a companion to reading Being and Time, you might listen to Hubert Dreyfus' lectures. https://archive.org/details/Philosophy_185_Fall_2007_UC_Berkeley - I find them, as with other secondary sources, to often be quite skewed, but if you're reading along with Heidegger, they can make the terminology clear, so they're helpful for the first time through the book. My favorite piece of writing about Being and Time is Derrida's Aporias - this is also a difficult work - but quite short, and worth taking a look at after you've read Being and Time.