How to make select mandatory? and also select by default the first available option?

Question

I have a system in php that generates selects according to the database information, in case I would like to make these selects mandatory because if they do not go unnoticed and also always leave the first option selected as default.

below the code tree where the selects are created:

if(get_field('atributos')){
    $selects = array();
    $i=0;
    $atributos = explode(PHP_EOL,get_field('atributos'));

    foreach($atributos as $atributo){
        $opcoes = explode(':',$atributo);
        $name = $opcoes[0];
        if(!empty($opcoes[1])){
            $opcoes = explode(';',$opcoes[1]);
        }
        $selects[$i] = array('name'=>$name,'options'=>$opcoes); 
        $i++;