Are there tools available to help make my site ADA compliant?

I'm helping a friend with a website for her eye clinic. I'm interested in making the website accessible for people with vision problems. Is there anything special that I can do to make sure that people can use screen readers and change the font size along with the font and background color to their liking? From what I've read, each person with a vision issue can have a different preferred setting or color that they can see better. This is from the ADA.gov:
"Websites should be designed so they can be viewed with the color and font sizes set in users’ web browsers and operating systems. Users with low vision must be able to specify the text and background colors as well as the font sizes needed to see webpage content."

I've also read that proving alt tags for images is helpful, but there is also a  “longdesc” tag for images that need a longer description. Is there a simple way to add a longdesc tag?

Another tip from the ADA is a include a “skip navigation” link at the top of webpages that allows people who use screen readers to ignore navigation links and skip directly to webpage content. Is it possible for me to incorporate that into my site?

Is there a way to add a descriptive html tag to buttons?

Thank you!