The Caesar cipher shifts every letter by a fixed number. Suetonius tells us Caesar used a shift of three. It's trivially breakable — and that's exactly why every cryptography course starts with it.
Lesson 1: key space. Only 25 keys. If an attacker can try every key, your cipher is broken. Modern ciphers have 2^128 or more.
Lesson 2: patterns leak. Caesar preserves letter frequencies; E is still the most common letter, just renamed. Every substitution cipher has this flaw, which is why frequency analysis — described by al-Kindi in the 9th century — beat them for a thousand years.
Lesson 3: Kerckhoffs's principle. Security must rest on the key, not on keeping the method secret.
Lesson 4: polyalphabetic ideas. Vigenère uses many Caesar shifts in turn to flatten frequencies — a step toward modern design.
If you want to try these hands-on, caesarcipher.org has free in-browser tools: a Caesar encoder/decoder, frequency analysis and a cipher identifier. No signup; everything stays in your browser.