Start with Kant's prolegomena.

Also, read Bennett's version. Autistic snobs will tell you to read the original. I always reply to them that unless you're fluent in German, you're already not reading the original. Bennett's project is to translate texts not only into English, but also into a contemporary dialectic. Kant's a very difficult read so what most university students do is clumsily waffle their way through it, understanding virtually nothing, and then remember the professor lecture as well as they can. The professor's lecture is generally a shittier version on Bennett in that it's a professional philosopher turning 18th century German into 21st century English.

I have a degree in philosophy and when I took my first class on Kant, I accidentally read the Bennett version because I just googled for the PDF of the assigned texts instead of buying the book. Everyone thought I was some sort of super genius for being able to understand him so easily and explain him so clearly, when in reality I was just using better study materials. I aced the class with minimal effort, having lost absolutely nothing by not reading the original. I only learned who Bennett was when writing the final essay when the professor let me know that my citations were not to the original text. She said Bennett's a good philosopher though and that learning Kant the way that I accidentally did was not inherently bad, but provided some snobby justifications for suffering through the "original" book.