Type on youtube "Catia V5 tutorial" (I assume you have V5, it is the most common version), whichever video comes up in the first five, that is the better one. Look for basic tutorials if you are completely new in CATIA. Via youtube you can become advanced in CATIA, it depends how much you are willing to learn. My recommendation is https://www.youtube.com/watch?v=R8YEA4e9hm4&list=PLkMYhICFMsGbYCvbGrrygtqGiBGguIzbf
on this link there are a lot of tutorials for beginners.
Tutorials are nice for learning something quickly, but often you don't learn very much. And many of the tutorials are not professionally written, so the quality is "hit or miss."
If you seriously want to learn CATIA (or any CAD system), the best way to to take an instructor-taught class. Preferably from a a Dassault Certified instructor.
A good option (in my opinion) for self-teaching is I.Get.It. They offer a full curriculum of CATIA and several other popular CAD systems. Good for the beginner, as well as the advanced user. A little expensive, but well worth it to learn how to use CATIA quickly and the right way. www.myigetit.com