Hi Jackson,
Many times we start letter by writing, to whom it may concern. This is not an accurate statement. I would say that we rarely start a letter with those words. I don't even remember the last time in my life that I did that.
If you search the Forum with this expression, you'll find information on the rare and special occasions that this may be appropriate. However, my advice to you is not to even consider using this form of address.
I think you are probably wondering how to start a letter to someone if you don't know their name. That's a very different question. Possible choices include 'Dear Sir or Madam', among others.
Best wishes, Clive