As with most things in philosophy, I would suggest starting with the Wikipedia article and the Internet Encyclopedia of Philosophy article on the topic. These usually give a good, broad overview of the topic.

I would also suggest finding a commentary that you like. I can't say I'm too familiar with Heidegger or commentaries on his work, but a quick google search shows several commentaries and outlines that seem useful.

As with most things in philosophy, I would suggest starting with the Wikipedia article and the Internet Encyclopedia of Philosophy article on the topic - not as a replacement for reading the text, but more to get a sense for an overview of a text and the context in which it was written. In many texts today, you will often see front-matter or appendixes that give similar sorts of overviews.

I would also suggest finding a commentary that you like. I can't say I'm too familiar with Heidegger or commentaries on his work, but a quick google search shows several commentaries and outlines that seem useful.