I am undergraduate in Aerospace engineering and I want to learn CAD so can you list some good CAD software (free and paid) helpful for airplane designing?
I have friends involved in the aerospace industry as part of an internship via Embry Riddle Aeronautical University. Almost exclusively, the curriculum involves using Catia for design and engineering. The internship is with a big company in the western USA and it uses that software for everything involved in design. It's not an inexpensive program, but students either get it free or at a substantial discount, I can't recall.
Other programs you could learn may provide some experience with process development and design practices, but may hinder your transition if you later require to use Catia.
If you are currently a student in this field, you should have been presented with appropriate technology as part of the program.