Courses
There is so much educational material available online that it can be a bit overwhelming. I've tried here to point out some good places to start. All of the following links are to full courses (some short, some long) that will help you understand a particular topic.
Guides
Beginner's Guide to Python (on the official Python site – lots of helpful links)
Google's Python Class - excellent introduction to Python if you have some programming experience
JavaScript tutorial on W3 Schools site (easy to start learning, but quite comprehensive)
Google Apps scripts tutorial - learn how to add functionality to Google docs and sheets
Pydon'ts - a series of articles for advanced Python programmers
Competition and coding game sites
Short courses from Kaggle
Intro to Programming (no experience required) - 5 hours
Python - 5 hours
Intro to Machine Learning - 3 hours
Intermediate Machine Learning - 4 hours
Pandas - 4 hours
Data Visualization - 4 hours
Feature Engineering - 5 hours
Data Cleaning - 4 hours
Intro to SQL - 3 hours
Advanced SQL - 4 hours
Intro to AI Ethics - 4 hours
Intro to Deep Learning - 4 hours
Computer Vision - 4 hours
Geospatial Analysis - 4 hours
Machine Learning Explainability - 4 hours
Microchallenges - 1 hour
Natural Language Processing - 4 hours
Other courses
HTML & CSS crash course - Lighthouse Labs
Udacity courses (some of these may not be accessible)
Shell Workshop (beginner level - 1 day) ud206
Linux Command Line Basics (beginner level - 1 week) ud595
SQL For Data Analysis (beginner level - 4 weeks) ud198
Intro to Relational Databases (intermediate level - 4 weeks) ud197
Introduction to Python Programming (beginner level - 5 weeks) ud1110
Version Control with Git (beginner level - 4 weeks) ud123
Design of Computer Programs with Peter Norvig (advanced level - 2 months) cs212
Intro to Algorithms (intermediate level - 4 months) cs215
Intro to Data Structures and Algorithms (intermediate level - 4 weeks) ud513
Introduction to Machine Learning (intermediate level - 10 weeks) ud120
Intro to HTML and CSS (beginner level - 3 weeks) ud001
HTML5 Canvas (beginner level - 2 weeks) ud292
Intro to JavaScript (beginner level - 2 weeks) ud803
Responsive Web Design Fundamentals (intermediate level - 2 weeks) ud893
JavaScript and the DOM (intermediate level - 4 weeks) ud117
Intro to jQuery (intermediate level - 2 days) ud245
ES6 - JavaScript Improved (advanced level - 4 weeks) ud356
MOOCs
Coursera - Interactive programming in Python - fun university level course from Rice University
Coursera - Algorithms - university level course from Princeton
Coursera - AI For Everyone by Deep Learning - Andrew Ng
Coursera - Mathematics for Machine Learning by Deep Learning
EdX - Harvard CS50 - introduction to HTML, CSS and web programming using Python
EdX - Introduction to Computer Science and Programming Using Python - excellent MIT course
EdX - How To Code: Simple Data - UBC first year CS course
EdX - AP Computer Science A - Purdue University
MIT - Linear Algebra - Full MIT course taught by Gilbert Strang