How do I begin making my WordPress website more accessible?

There are many tutorials online. Here are some examples to get you started, including one from WordPress and one from the University of Michigan:

• https://make.wordpress.org/accessibility/useful-tools/
• http://webaccess.hr.umich.edu/
• https://www.webucator.com/

Once you've made some adjustments, you can test the accessibility of your website with online tools such as http://wave.webaim.org.