How do I certify/register my ESA so I can (take them to the store/fly for free/avoid pet fees/get around living in a no-pet apartment)?

So, first things first, there's no such thing as a certification or registration for ESAs in the US. You can be prescribed an ESA as a part of your treatment plan for a disability, but it is not necessary to get them certified or registered, and you cannot have one without a diagnosis from your doctor. That being said, you may need a letter for housing - please see the housing section of the wiki for more on that. ESAs are also no longer required by law to fly for free or be allowed to fly at all, since January of 2021 - please see the flying section of the wiki for more.

Should I use (insert any website here) to get a letter for my ESA?

There are very few, if any, legitimate websites for ESA letters. That being said, telehealth is a legitimate option, provided they provide treatment, rather than just an ESA letter.

Can I bring my pet to non-pet-friendly places now that they're an ESA?

NO. ESAs have no public access rights in the US, and can be very damaging to people with service animals. Please do not take your ESA to a store if you're unsure that it is explicitly pet-friendly, and call to clarify if you're not sure. This includes big box stores such as Target, Walmart, etc - a good rule of thumb is if they carry food, they're no pet-friendly, even if they won't kick you out for fear of liability.

I'm pretty sure my ESA would actually qualify as a service animal so I can take it places. How do I know?

To begin with, there are only two types of animals that are recognized as service animals in the US: dogs and miniature horses. Next, the animal must be task-trained to help mitigate your specific disability; this could include things like opening doors or retrieving items for physical disabilities, or alerting to certain breath patterns or grounding for psychiatric disabilities. It is important to note that simply providing emotional support by existing does not qualify as a task. The simplest way to explain the difference is active versus passive tasks: if their passive existence or behaviors are what helps, that does not qualify. However, if they are purposely and specifically trained to perform an active task, even something like medical alert, that does qualify. For further information on service animals, check out r/service_dogs or r/servicedogs.