The most comprehensive web accessibility resources are from W3C (World Wide Web Consortium) WAI. It’s a large website with numerous links and tools. Here are some specific pages that should be helpful for getting started.

Introduction to Web Accessibility to learn the what and why
Tips for Getting Started for basic considerations
Essential Components for more of a conceptual view
Perspectives has videos that demonstrate the benefits of web accessibility design and related technologies
Easy Checks to assess the issues on your site
Evaluation Tool List is alphabetic and very comprehensive. The WAVE evaluation tool is commonly used and has two forms: Online WAVE tool or WAVE browser extensions
Tutorials – See the right sidebar menu
WCAG 2.0 at a Glance summarizes the four principles of POUR
How to Meet WCAG 2.0 is organized by the four principles

How to Ensure Accessibility for Every Online Audience is an excellent read on why this matters so much.

Tools to Support Learning Science Strategies