How to assign a shortcut key (something like Ctrl+F) to a text box in Windows Forms?

One way is to override the ProcessCMDKey event.

protected override bool ProcessCmdKey(ref Message msg, Keys keyData)
{
    if (keyData == (Keys.Control | Keys.S))
    {
        MessageBox.Show("Do Something");
        return true;
    }
    return base.ProcessCmdKey(ref msg, keyData);
}

EDIT: Alternatively you can use the keydown event - see How to capture shortcut keys in Visual Studio .NET.

Capture the KeyDown event and place an if statement in it to check what keys were pressed.

private void form_KeyDown(object sender, KeyEventArgs e)
{
    if ((e.Control && e.KeyCode == Keys.F) || (e.Control && e.KeyCode == Keys.S)) {
        txtSearch.Focus();
    }
}