How can i create a function tag in my plugin

Question

You are looking for Enclosing Shortcode. Here is an example of how you can achieve this.

function example_shortcode($atts = [], $content = null)
{
    // do something to $content

    // run shortcode parser recursively
    $content = do_shortcode($content);

    // always return
    return $content;
}
add_shortcode('example', 'example_shortcode');

Now you can use it like [example] Here is my content [/example]