I would definitely start with listening to the free audio i linked to. As to reading, I wouldn't start wtih Institutes of Biblical Law. That's way too much. I would probably start with something like Foundations of Social Order or Intellectual Schizophrenia. My personal favorite is Politics of Guilt and Pity
Foundations of Social Order is world changing, it's often too much for a new person to digest. It was the first book I read on Rushdoony, it took me about a year, as my soul kept feeling like a dartboard. Often I could read a few pages and then spend all of the next day thinking about it, as I started analyzing it. I've since read it several times now, just to actually digest it's full meaning, as I slowly became Reformed over a period of about 15 years. Each time I read it, it's new light.
Rushdoony's entire way of comprehensive thinking in terms of Reformed Theology is what is most impressive and what is incredibly value to glean from him, just as much as the theology he is teaching applied to a specific sphere. If you want to learn how to think, read Rushdoony.
All of his works are excellent as I've read them all, I've also had the privilege of reading private correspondence between Rushdoony and my Pastor during the 1970's, and several legal analysis of Rushdoony, that has been incredibly valuable. I've taken up some research on this and will be going to St Louis sometime soon and hope to go the Reformed library of the PCA and read some letters between Rushdoony and Singer deposited there.
For a new guy that has never read him, I would recommend "The Atheism of the Early Church," because it is small, a hundred pages or so. This is where I start my children when they get about 12 or 13 years old on Rushdoony, it's a small enough work to get started without being intimidated by the magnitude of what lays ahead when you get through 10 pages of a 400 page book.