select one record in dataGridView and see fields in textBox

Let factorize your question into two parts

1) Display data from database into datagridview in C#

Read this article

How to populate DataGridView, GridView with SQL statement in C#[^]

2) Populate textboxes depend upon selected row in datagridview in C#.

private void dataGridView1_CellMouseClick(object sender, DataGridViewCellMouseEventArgs e)
{
textBox1.Text = dataGridView1.Rows[e.RowIndex].Cells[0].Value.ToString();
}

Hope this helps if yes then accept and vote the answer otherwise revert back with your queries
--Rahul D.