I began studying the Critique 35 years ago. I am still studying. Be careful when reading critical analysis by others who claim authority on Kant's Critique or Prolegomena. Research whether they are pro or anti Kant - however tacit.I am not suggesting how to select, only be aware that even philosophers can be sceptics, dogmatists, realists, idealist or a bit of each. I will emphasize a previous suggestion that before attempting Kant you should have a thorough handle on philosophic terminology and concepts, analytics, logic and some history from at least Descartes to Kant. Roger Scruton's 'Modern Philosophy' comes highly recommended. Plus there is GOOGLE, an Invaluable resource. Choose your reading material carefully or you will find yourself wandering aimlessly down dark dead end alleys lost and confused wandering in circles cul-de-sac after cul-de-sac wasting vast amounts time getting nowhere.
Authors to consider : (5)Allison -'Transcendental Idealism' (defends T.I with reservations)
(1)Gardner -'Kant and The Critique of Pure Reason' (Excellent Guidebook)
(6)Grier- 'Kant's Doctrine of Transcendental Illusion'- In defence of -
(7)Stern - Transcendental Arguments and Scepticism (Negative)
(4)Strawson - 'The Bounds of Sense'(Common Sense REALIST-anti T.I)
(8)Langton-'Kantian Humility'- Epistemic vs metaphysical a-priori knowledge
(3)Guyer- Kant(Routledge)- Subtle negativity on many Kantian premises.
(2)Guyer-The Camebridge Companion To Kant- 14 essays by different authors
(9)Guyer-Kant and the claims of knowledge- Confused representation of Kant's 'refutation'-Idealism and Realism (Epistemology)
I found Gardner's Guidebook the best starter. From there the choice of paths to comprehending the depth and breadth of Kant is yours. It is an addiction. Lifetimes have been spent in this study. Tread carefully. It all depends on how much time you have, how much patience and determination you have and how much you are 'awed' by the unparalleled genius of Emanuel Kant.
These opinions and suggestions are mine and come from personal experience. Other suggestions and criticisms must be equally considered as I do not claim to be an authority on Kant, only a guest.
The numbers in front of the book suggestions indicate the order I read them in. There are countless others that could be read aside from these.
I do think GARDNER is by far the BEST place to start. This is not to suggest you do the same.