Reddit reviews Theories of International Relations

Slaughter wrote a very concise summary of the main IR theories.
Simply look-up and read the main articles referenced in that text/of the authors, and you'll have a solid entry into the theoretical backbone to IR.

With regard to textbooks: I read The Globalization of World Politics before I started my studies and felt it was quite accessible as it split up the main theories, mid-range theories, as well as different issue-areas into nice digestible chunks in a very accessible manner.

If you enjoyed the Slaughter summary and want to truly dig into the academic side of it all: Theories of International Relations was my favourite IR textbook. I got to admit that quite some of our class thought it was at parts too dense, but that is exactly what I was looking for. Given that you will be doing IR for at least three years, this book should come in useful more than just once.