The first inclination is to create a shortcode that will look like [MyShortCode]My Only Parameter[/MyShortCode], which would look like this
function MyShortCode_func($content = null){
//————-stuff happens
return $processedContent;
}
However, if you want to have parameters = you must have the $atts parameter in the function, even if it is not used. The proper function would be
function MyShortCode_func($atts, $content = null){
//————-stuff happens
return $processedContent;
}