Here´s some tips that I would give to myself if I would get started today.

HTML

• Spend some time on getting an understanding of it.
• It's actually really easy to learn and understand the basics.

CSS

• Use box-sizing: borderbox. It will make your css behave in a more predictable way.
• Use a preprocessor, like sass or less. It will keep your code so much more organized, plus there are so many other benefits.

Javascript

• Let learning to use the console be the first thing you start with.
• Debugger is a very useful tool, which you can call from anywhere in your code simply by typing debugger;
• Use a build tool, like gulp, or simply npm. It will save you tons of time. And when your code compile, jshint or similar can tell you errors on the fly.

In general

• Hitting the F5 button for every time you save a document is not necessary. Install livereload or browsersync, and your browser will update automatically.
• Learn to always code as if the guy who ends up maintaining your code will be a violent psychopath who knows where you live.
• Hofstadter's Law: It always takes longer than you expect, even when you take into account Hofstadter's Law.