How to activate (unlock) my cellphone number by AT&T

Unlock is the wrong term. 

If you're coming to the United States you will be assigned a new Us phone number when you sign up and pay for service

I assume you'll be starting prepaid services if you are only coming for a visit., even if you are intending to stay in the United States it's unlikely you will qualify for postpaid as a new resident. This is done after you get to the United states. Prepaid service does not work in Brazil and postpaid service does not roam for the first 60 days.

You will probably need to purchase a new phone, not all phones are capable of working on AT&T

Hello @joaobotaro, we can point you in the right direction to activate your number once you are back in the US.

We do not currently have international locations where you would be able to get the service activated.  We are able to activate the SIM over the phone. The account owner will need to contact us. Please make sure you have the SIM and IMEI of the device to be activated.

You can reach out to the AT&T international department at 1-314-925-6925. You can call from your AT&T device for free while traveling. 

When you are back in the US you can follow these steps to activate your services:

1. Go to Device Support.
2. Choose your device.
3. Scroll to Getting started and select Hardware & phone details.
4. Choose Insert or remove SIM card and follow the steps.

Thank you for reaching out to AT&T Community Forums! 

If you have any further concerns please reach back out to us.

Danielle, AT&T Community Specialist 

First of all, thank you!!

But will I have to activate my number pretending that I'll be starting a postpaid service? By authorizing it on the app or making a call to make my number work.

you cant activate a foreign number in the US you can only roam with a valid phone model and a roaming plan with your countries provider 

Yes, I got it, but I want to know how can I activate my AT&T number.

You cannot activate and use AT&T service until you are in the United States