how do I delete a file using onclick

Can anyone tell me how to delete a file with onclick? I thought I could use the following code, but it turns out that the file was deleted upon every page load regardless of whether the reset button was clicked.

<?php
$triggerfile = "wp-content/themes/headway-208/ideas.docx";
if (file_exists($triggerfile)) {
echo '<input type="button" value="Reset" onClick="'.unlink($triggerfile).'">';
} else {
    echo "The file $triggerfile does not need to be reset";
}
?>

I’ve been told I need to use Ajax, although I’m not familiar with Ajax at all. I did find this thread which might steer me in the right direction.
https://www.phpfreaks.com/forums/index.php?topic=159544.0
However, given that this is all located on a wordpress page, I’m not sure how to implement everything. Specifically, where does the ajax.php go?

I may be going about this wrong. If so, please somebody turn me in the right direction or direct me to some documentation or code that would be helpful.

Thanks much!