Use the :nth-child
pseudo selector or with :first-child
(in your specific case of first column)
td:nth-child(1) {
background: red;
}
<table>
<tr><td>a</td><td>1</td></tr>
<tr><td>b</td><td>2</td></tr>
</table>
In your specific case it's also doable with:
td:first-child {
background: red;
}