Eric
Inspect the source code of pretty much everything. Tamper with pages through the browser Developer Console. If anything remotely catches your attention on the web use the Dev Tools to figure what makes it tick. Do that alongside all the other recommendations here - it's good to read manuals but it's great to see things in the wild.
Mark Dain
Anywhere except w3schools; tons of outdated resources and bad examples. Avoid like the plague. Due to various SEO reasons they're usually #1 on Google but MDN is #2. Go there instead.