Any EU national can work in Portugal. People coming to Portugal from outside the EU must obtain a work permit.
The work permit (Autorização de trabalho) can only be issued if you have been offered a contract by a future employer. You will also need a visa that allows you to work in Portugal. To obtain a visa you should ask the Portuguese Immigration Office .
Irrespective of your country of origin, you will need the following to work in Portugal:
A Residence Permit (Cartão de Residência). You can get this at the Portuguese Immigration Office .
Registration with Social Security (foreign workers must contribute to Social Security).
A Tax Number. You can get this by going to your local Tax office (Direcção-Geral dos Impostos) also known as DGCI .
Many professionals will need a special license or certificate to practice their trade in Portugal. As in most countries, this is the case for medical practitioners, lawyers and many other high-end jobs. However, Portugal also requires all self-employees and most trades to be certified. If you are a plumber in your home country you may not be able to work as a plumber in Portugal.
To find out how your professional status might be affected by your move to Portugal you should look at NARIC's website which gives advice on professional skills across the globe.