Well, it’s not recommended to make your own crypto systems for many reasons, but to make your own system,you need to learn the basics of existing crypto systems first.
So I recommend you this: https://www.coursera.org/learn/crypto
and practice in CTF crypto challenges to see codes and their vulnerabilities, so you learn programming and new crypto things.
Also I just have started a course called CryptoMathesis which will teach many crypto systems and will learn people how to program them in python3.
You can check this out: CryptoMathesis - In depth Cryptography Tutorials
(The only disadvantage is that it’s in greek,but I explain things very simply that you can understand them. Also I add english subs)
For now I have explained RSA and this week I will publish how AES is working. 