What are the coding guidelines or samples to make a website compatible or create a new website with Web Accessibility AA standards?

I have got a new project where we need to do a responsive web application in which i have got the part of creating the HTML web pages using HTML, CSS, Bootstrap etc., in addition to creating the website i need to take care of the accessibility AA standards.

I am currently studying and exploring about Accessibility but not so much sure that how to implement it along with the HTML creation.