The 'I think'
There is a version of the "Cogito" ['I think, therefore I exist': GT]
that Kant is found endorsing, but not in the manner intended by
Descartes. At B132 [of the Critique of Pure Reason] he says that,
It must be possible for the "I think" to accompany all my
representations, otherwise, something would be represented in me
which could not be thought at all, and that is equivalent to saying
that the representation would be impossible, or at least would be
nothing to me.
Thus, the "I think" is a necessary condition, if an object is to be
thought at all. It is not a Cartesian device designed to defeat total
skepticism. Consciousness accompanies all thought, of course, and
"immediately includes in itself the existence of the subject; but it does
not so include any knowledge of that subject, and therefore also no
empirical knowledge, that is, no experience of it." (B277)
(Daniel N. Robinson, 'Kant's 'Seamless' Refutation of Idealism', The Review of Metaphysics, Vol. 64, No. 2 (DECEMBER 2010), pp. 291-301: 297-8.)
Roughly, Descartes used the fact of his thinking to 'prove' (by steps not necessary to explain here) that thinking was his essential nature; he was essentially a thinking being.
Kant's counter-position is to say that yes, when I am conscious and have various 'representations' - experiences, perceptions - then I am aware that I am the subject that has them. I am aware of them as mine. That, however, tells us nothing about the nature of the 'I'. I have no insight into the nature of the subject that has experiences even though I am that subject.
Back to Kant
The best course, if you do not have time or inclination to read Kant's Critique of Pure Reason, is to read a summary or commentary. Roger Scruton's *Kant - A Very Short Introduction' (Oxford) would I think be of help.