If you're writing a Complaint Letter to a business or corporation that you don't have a direct contact for, you'll need to obtain a mailing address or email address to send your letter to.
Most companies list general mailing addresses on their website, and/or list contact information for people that hold higher positions in the company, like management and executives. You can make use of this information to decide who is the most appropriate person or department to send your letter to.
For instance, if your complaint is regarding a phone conversation you had with a customer service representative who you feel behaved inappropriately towards you, the most appropriate person to direct your Complaint Letter to would likely be the customer service manager.
If your complaint is about a defective product that you've been unable to get replaced or repaired, you may want to address the product or service manager, or an equivalent position.
Essentially, the closer you are able to pinpoint the person that is most likely to help solve your issue, the greater the chance your Complaint Letter will lead to a timely, successful resolution.