what program do i use to write a letter?

TextEdit is built into OS X

otherwise as stated Pages is avaialbe from the Apple Store

Open Office is a free word processor from Apache

Apache OpenOffice - Official Site - The Free and Open Productivity Suite

Libre Office is also free

https://www.libreoffice.org/

Microsoft Office is the industry standard

Buy Microsoft Office 365 Home & Personal subscriptions

https://help.apple.com/pages/mac/6.3/Try using pages.