How to sum values in a table column to each record typed

I need to add in the columns total_débitos and total_créditos of the database table, the value of the posting of each of the records entered. I have the following code:

In the table:

saldo_anterior   total_débitos   total_créditos   saldo_atual

My code:

<tr>
<td align="right">
    <strong>Valor do Lançamento R$:</strong>
</td>
<td>
    <input name="valor_lancto" type="text" id="valor_lancto" size="14" 
     maxlength="20"/>
</td>
</tr>

For each rollout, add in the total_débitos or the total_créditos column, depending on the type of posting (if [D] debit or [C] credit)