First time I'm going to use Ajax with Craft CMS therefore I'm still confused how it works.

What I need is this, when I click on a specified link in the front, I need the CMS to load some entry/entries and display the data in a popup on the same page of course.

The CSS for the popup is ready, the HTML too (static for now, not twig template) the backoffice also is ready. The only thing I need is how to get the entries (basic explanation if possible) with ajax (It seems to be the best solution) and load them into the page then show them in a popup.

Is there any link(s) to help in my task or someone to explain those first steps please ?