Assuming you mean populating the text box with what is selected in data grid view, use the CellClick or CellEnter events on the data grid view. CellEnter is more useful as it will trigger on cell click and when a cell is focused (i.e. using arrow keys to navigate in the grid).
private void dataGridView1_CellEnter(object sender, DataGridViewCellEventArgs e){
object data = dataGridView1.Rows[e.RowIndex].Cells[e.ColumnIndex].Value;
txtCarga.Text = data.ToString();
}