There is tons of information on how to create accessible content. That is not what this guide is. However, here are some useful tools to help you evaluate the accessibility of your content:
WAVE (Web Accessibility Evaluation Tool): a popular and useful tool for testing a website's accessibility. It provides specific, marked recommendations based on what is wrong with a site.
W3C Accessibility Evaluation Tools List: a page maintained by W3C's Web Accessibility Initiative that lists tools and their uses.
Section 508 Government Accessibility Testing Guidance: a resource by the General Services Administration for testing for accessibility.
Software Testing Help's Accessibility Testing Guide: an independently produced guide for how to test for accessibility, including the relevant law and tutorials.
If all else fails, a consultant wrote a handy list of accessiblity tests you can run with no tools but yourself.
For dynamic content (anything past a basic text page) you may consider testing using a screen reader with your content. That way, you will see any clear issues with it that you might be able to improve for your audience. There are not many good free options (it is a problem), so here is one.
NVDA Screen Reader: an open-source screen reader that is free to download and use.
Test on multiple types of devices (like a phone, a computer, and a tablet). Always remember that content on a screen can vary widely based on screen size.