Front End
Learn HTML, CSS, JavaScript​
I would recommend just reading the corresponding MDN page for picking up CSS and HTML. You mainly want to understand how they work together to organize and style a webpage. Links here:
JavaScript is rather simple as well, the langauge has an interesting history but has now become the building block of the internet. It was built in 10 days so you can imagine there are bugs (well known typeof null bug) that were remainances from that time.
NPM​
NPM is an important package directory used to manage dependencies in most modern web apps