You can find by use JQuery
$("#GridviewID tbody").find('tr').each(function (i) {
var text = $(this).find("input[type='input']")[0];
});
The above loop will find each row of grid and the below code is to find the first textbox in the row.
var text = $(this).find("input[type='text']")[0]
or u can write as below
var firstRow = $("#GridviewID tbody").find('tr')[0];
var firstTextbox $(FirstRow)..find("input[type='text']")[0];