How do I make a textbox always right when I'm focused?

A simple way to resolve this is if the value is 0.00 you clear the field when there is a focus on it.

document.querySelector('input[type=text]').addEventListener('focus', function () {
    if (this.value !== '0,00') return;
    this.value = '';
}):

The same code using jQuery :

$('input[type=text]').on('focus', function () {
    if (this.value !== '0,00') return;
    this.value = '';
}):

There are other ways you can select all the content of the field so that anything typed will overwrite it altogether, for example:

document.querySelector('input[type=text]').addEventListener('focus', function () {
    this.selectionStart = 0;
    this.selectionEnd = this.value.length;
}):

Do not forget to replace the input[type=text] selector with one that fits your code, which is ID, class, or other attributes that identify the field (s) you want to apply the event to.     

You can also use jQuery and also use the maskMoney , see an example

$("#demo3").maskMoney({prefix:'R$ ', allowNegative: true, thousands:'.', decimal:',', affixesStay: false});

Just use the PHP equivalent number_format() for Javascript.

$("#omeuinput").keypress(function(e) {
var valor = $(this).val();
valor = number_format(valor, 2, '.', '');
$(this).val(valor);
});


function number_format(number, decimals, dec_point, thousands_sep) {
number = (number + '').replace(/[^0-9+\-Ee.]/g, '');
var n = !isFinite(+number) ? 0 : +number,
    prec = !isFinite(+decimals) ? 0 : Math.abs(decimals),
    sep = (typeof thousands_sep === 'undefined') ? ',' : thousands_sep,
    dec = (typeof dec_point === 'undefined') ? '.' : dec_point,
    s = '',
    toFixedFix = function(n, prec) {
        var k = Math.pow(10, prec);
        return '' + Math.round(n * k) / k;
    };
// Fix for IE parseFloat(0.55).toFixed(0) = 0;
s = (prec ? toFixedFix(n, prec) : '' + Math.round(n)).split('.');
if (s[0].length > 3) {
    s[0] = s[0].replace(/\B(?=(?:\d{3})+(?!\d))/g, sep);
}
if ((s[1] || '').length < prec) {
    s[1] = s[1] || '';
    s[1] += new Array(prec - s[1].length + 1).join('0');
}
return s.join(dec);
}

Elements html have triggers that are triggered by events. In the element of your form where the monetary value will be entered, you can set up an action to run every time the content of an item changes.

For this you will use the onChange feature

javascript function to format monetary values

<script type="text/javascript">
    function formatMoney(c, d, t){
    var n = this, 
        c = isNaN(c = Math.abs(c)) ? 2 : c, 
        d = d == undefined ? "." : d, 
        t = t == undefined ? "," : t, 
        s = n < 0 ? "-" : "", 
        i = parseInt(n = Math.abs(+n || 0).toFixed(c)) + "", 
        j = (j = i.length) > 3 ? j % 3 : 0;
       return s + (j ? i.substr(0, j) + t : "") + i.substr(j).replace(/(\d{3})(?=\d)/g, "$1" + t) + (c ? d + Math.abs(n - i).toFixed(c).slice(2) : "");
     };


     function formatar(){
        valor=document.getElementById("valor").value;
        valorFormatado = formatMoney(valor, '.', ',');
        document.getElementById("valor").value = valorFormatado;

     }
</script>

In your text field

    <input id="valor" type="text" onchange="formatar()">

Reference about formatMoney