checkbox (check and uncheck) changes the dropdownlist dynamically using AJAX

I am new to javascript, jQuery and AJAX (Frontend) technologies. I have been working on backend. I need to create a dynamic page where, the page has 4 checkbox options and one dropdown list. Once I select the checkbox the dropdownlist should show dynamically depending on the checkbox selected. If I select 2 checkboxes, I need to show combined of both the list in the dropdownlist. Can anyone please guide me how to proceed or any sample code or reference I can look in it. Thank you in advance.