Now, if 100 people answer, you are likely to get 150 recommendations, so here is my mine, honed from many years experience in both home and professional cooking:
Cast iron. Get yourself 3 cast iron skillets: 8-inch, 10-inch and 12-inch to start. You can expand your collection as you develop more interest in cooking and a love for cast iron cooking.
Stainless. I have no idea what brands are available in Portugal, but what you want is a selection of different sized tri-ply pots and pans with lids.
The Cuisinart Contour Stainless 13-Piece Cookware Set or something similar is a good starter set. Good quality at a reasonable price, and as with the cast iron you can build upon it as your experience and needs require because Cuisinart sells different cookware singly. You might want to make sure this cookware is induction capable, just in case there is an induction cook top in your future.
Non-Stick. What I do, and what I always advise others to do, is go to general merchandise store like Target and pick up a 3-pack of non-stick frying pans. The 3-pack sells for $18 to $20 USD. I use them until they get scratched or the non-stick wears out, then throw them in the recycle bin and buy another 3-pack.