Here’s a search of the CCC on Original sin and I hope it helps. Be sure to have a look at the context of the sections it refers to as it often clarifies things even better. Here’s the Link to that search engine for future reference.
The 3rd chapter of the Gospel of John is a good place to begin. Why would one need to born again if we are not born in sin?
Psalm 51 is also a relevent passage, noting especially verse 7.
St. Paul get’s into this also in Romans 5 and that is a good context as well. Verse 12 says plainly: "12 Wherefore as by one man sin entered into this world, and by sin death; and so death passed upon all men, in whom all have sinned. " So there’s your original sin right there.
This article from This Rock is very good.
John Martignoni has a great article HERE on infant baptism and original sin.
Greg Mockeridge’s article is also good.
The ECF on Original Sin
There are 2 good questions and answers on this page about original sin that might also help. (The rest of them ain’t bad either, in fact.)
Feel free to PM me if you need anything else.