how to set font in notepad by using c#

You can change the default font in notepad with the following registry keys (this will set to "Courier New" and size of 9):

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\Software\Microsoft\Notepad]
"lfEscapement"=dword:00000000
"lfOrientation"=dword:00000000
"lfWeight"=dword:00000190
"lfItalic"=dword:00000000
"lfUnderline"=dword:00000000
"lfStrikeOut"=dword:00000000
"lfCharSet"=dword:000000b2
"lfOutPrecision"=dword:00000003
"lfClipPrecision"=dword:00000002
"lfQuality"=dword:00000001
"lfPitchAndFamily"=dword:00000031
"lfFaceName"="Courier New"
"iPointSize"=dword:0000005a

EDIT :
To change the registry using c# see here : Read, write and delete from registry with C#[^]

You don't have access to Microsoft Notepad, this is a proprietary isolated closed-source no-good application. Write you own editor; then the font won't be a problem.
Also, who needs Notepad? This a rudimentary language with feature below minimalistic requirements. Even the simplest home-made editor based on one of the TextBox controls (WPF or Forms) will do much better if implemented accurately.

—SA