I'd say one of the best advice a dev gave me, was that, I should not write duplicate code, but rewrite these parts to a single function.

And another one: If you use specific values in the code, instead of putting it in multiple places, assign it to a variable at one place and use the variable later on.

These advices sound quite trivial, but I think every beginner should learn these as eary as possible.
Boiiii have I seen shitty code from people who don't give a hobo's ass about maintainable code.

Be a good coder.
Write for quality, not quantity.
Care about your successor.

Thank you.
If not, I will fucking find you, fill your guts with napalm and light you up alive on a rusty pole while laughing hysterically.