HELP!!

I checked my mail today and someone had stuffed a live baby chicken in my mailbox!! I have no idea how old or what to do. I know it has to be at least 2 weeks old but that's it. No idea what type of chicken or what to do. Please help!!

You need to keep it warm, provide fresh water and some food.
You can get chick starter at the local feed store.
If you haven't done so visit the learning center at the top of the page for useful articles.
Here's one on raising chicks to get you started.

https://www.backyardchickens.com/a/raising-your-baby-chicks

Yikes! That's quite the dilemma you have there. Fortunately, when it is fully feathered, care will be very easy. For now keep it in a box or cage with a heat source (like a heat lamp). I keep my chicks outside when they are two weeks old but where I live is warm most of the year and of course with it being winter that's out of the question. Also do what Wyorp Rock suggests, and above all, don't panic. Chicks can be tougher than they look.

I'm sure everything will be alright.

If you are in the US that’s criminal, probably on several counts. I’d notify the post office. If it happened to you it could easily happen to others.

What do you want to do with that chick? They are social animals, it’s best to not try to raise it alone unless you have other chickens to eventually put it with. If you don’t want to keep it, I suggest you call your local animal control. See what they say.

It would not surprise me that someone figured they had a male and only wanted females so did this to get rid of it. I think your odds of that being a female and eventually laying eggs is less than 50-50.

In the meantime keep the chick warm enough so it’s not fluffed up and giving a plaintive peep. It might be a little hard to tell, because if it is alone it might be giving that plaintive peep because it is lonely. Give it water in a shallow bowl, a small shallow bowl so it doesn’t drown itself. In an emergency you can feed it with stuff you have there. Corn meal is excellent. It will provide enough energy and nutrition to keep it going until a better food supply can be found. One warning, do not give it anything that needs to be chewed up. Chickens don’t have teeth and rely on ricks in their gizzard to grind stuff up. That chick may not have any of those small rocks, called grit, so only feed it stuff that is already ground up.