There are dozens upon dozens of resources on accessibility, but these are some of the easiest to digest and a great way to start digging in.

Checklists and Guides:

• A11Y Project Accessibility Checklist – a FANTASTIC checklist to reference to help you hit all the quick wins. The rest of the A11Y Project site is pretty stellar, too.
• 18F Accessibility Guide provides an overview of requirements and a wealth of tools to help you dig in.
• WuhCAG provides plain language checklists of WCAG standards

Color Contrast:

• Tanaguru Contrast-Finder – checks your color contrast and provides close alternates that meet the guidelines.
• Contrast-Ratio – a lovely way to test color contrast, including opacity, and tweak settings yourself 
• WebAim Contrast Checker – when you want to keep it simple. Tests against both normal and large font sizes.

Do it With Drupal:

• Documentation: Accessibility tools and best practices for site builders is an overwhelming list of tools for Drupal site builders.
• D8 is serious about accessibility – take the pledge to build accessible with #D8AX 
• Drupal Accessibility group 

Simulators

• VisionSim lets you see the world through a simulation of vision disabilities (look it up -available for iOS and Android)
• NoCoffee Vision Simulator for Chrome lets you view the web through a filter

Accessibility Checkers

• Pa11y – free, open source, self-managed accessibility testing suite