get row by index

Using either the eq() function:

rows.eq(0).addClass('my_class');

Or the :eq() selector:

$('tr:eq(0)', tbl).addClass('my_class');

You could use the native rows^[docs] property on the HTMLTableElement.

$(tbl[0].rows[0]).addClass('my_class');

As noted by @Felix, I've assumed that tbl is a jQuery object. If not, do this:

$(tbl.rows[0]).addClass('my_class');