Belém is a big city in Northern Brazil.
There is only one highway leaving Belém, and that's BR-316. To hitch out, take a bus to Marituba. There are three big gas stations on the right side of the road, all within walking distance from each other, where a lot of trucks stop on their way out of the city. One of them is called Posto Oriente and has a friendly staff and cheap restaurant. Although there are other gas stations closer to Belém, many of them have security guards that might not let you stick around to ask for a ride.
You can take a bus at Presidente Vargas avenue, near of Praça da República. The buses that go to the road are "Ananindeua" or "Cidade nova", and you can ask for the rider to stop at a petrol station in Ananindeua.
The most part of the truckers don't leave the petrol station on sundays, so it isn't a good day to do hitchhike.