Starting with a list of essentials, here is a guide to starting out with web development:
OWASP Top 10
1 or more Server Side languages/platforms (PHP, Ruby, nodejs, Go, etc)
RESTful services, how to build and interact with
SOLID, OOP (Java tutorial, GoF design patterns)
at least 1 noSQL
some html templating language (mustache, handlebars, underscorejs, etc)
mvc/mvvm patterns and some SPA implementation of this (Angularjs, Emberjs, Knockoutjs)
A good place to start is http://www.w3schools.com/
that site has tutorials with an option to try each concept as you go. It's not a fully interactive tutorial, which is great because it also serves as a good point of reference.
Of course, the real heart of the web that we all know is based on HTTP. Here's a great way to understand the model. http://stackoverflow.com/questions/2001773/understanding-rest-verbs-error-codes-and-authentication
I once read a fantastic article about HTTP, but can't seem to locate it. That post on stack overflow has some good info. https://en.m.wikipedia.org/wiki/Client%E2%80%93server_model is sufficient for now to intro client-server.
StackOverflow is one of the best community-driven resources on the web. I'll say no more.
There are some great tools available for editing html and other types of code. I've found that Notepad++ works well for several languages. It's lightweight, and works for a variety of languages including semantic markups. https://notepad-plus-plus.org/
it is free and "easy to use".
What's Object Oriented Programming all about? Check it out on this Java tutorial https://docs.oracle.com/javase/tutorial/java/concepts/object.html
I cut to the chase, feel free to navigate back to the root and read the intro. Vest in Java or don't, but learn what they have to teach.
OWASP is a community focused on web security. They publish a wealth of information about how to protect your applications from being hacked - mitigate risk rather. You can find the top 10 threats and a wealth of other info listed on their site https://www.owasp.org/index.php/Main_Page
PluralSight is a hugely influential training site that has videos covering a range of IT related topics. They have some free videos, but a monthly membership gets you open access to all. This is a great way for more experienced developers to grow their knowledge base.