Hi there and welcome
To write a simple letter you can use Text Edit which is included in your system. Text Edit should be shown under your applications folder. Just open it up and start writing. Text Edit is a very basic word processor but it will do the job for you. If you want something more advanced, consider iWork of MS Office For Mac. They are both very worthy apps, but pricy and maybe more than you need.
If you would like any additional information or assistance, please do not hesitate to let me know.
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