Default focus on submit button

//write code on page load
Page.RegisterHiddenField("__EVENTTARGET", Button1.ClientID);

Code for select all checkbox in gridview on button click

for (int i = 0; i < GridView1.Rows.Count; i++)
{
CheckBox ca = (CheckBox)GridView1.Rows[i].FindControl("chkSelect");
ca.Checked = true;
}