Be sure you compare the various earplugs available, and try them out before the concert. They vary a lot in how much sound they attenuate. Not sure how things are done in Portugal, but in the U.S. at least, they have a decibel rating from OSHA printed on the package, which is a good start, but I've noticed a significant difference between plugs that have the same rating, so I always experiment. Also make sure you put them in correctly, as that makes a big difference as well.
To test, put them in and wait for them to expand. Then clap your hands softly at first and then gradually louder (don't go too loud too fast otherwise you can accidentally get too loud for yourself). Then once you're satisfied they're blocking enough, turn on your Hi Fi and see what music sounds like. Most people think music sounds terrible through earplugs when they first try, but I've found it's easier to get used to and still enjoy music then I expected.
There are choices there, of course: Most of the "musician" labelled plugs are not designed for maximum attenuation -- they're designed for a smooth response curve. And you can get custom moulded and fitted ones that sound really good. They're great (and worth the extra money) if you're not looking for the maximum protection and want good sounding music. I use them in moderate settings. For loud concerts I usually go with the foam ones I just get from CVS or Walgreens -- in my experience they block out the most.
Different earplugs also have a huge difference in what they do to your own voice. If you're at a concert and try to talk to your friend (or if you're the musician and try to sing), all earplugs will make your own voice seem louder, of course, but some way more so than others. In my experience, the rubbery or moulded ones seem to reflect my own voice too much for my comfort. The softer foam ones much less. Again, experiment.
Ultimately, you're limited in how much you can block out by the amount of sound that comes through your skull. As Tom CNYC commented above, that puts a maximum effectiveness limit on earplugs at about 40dB (in practice, they say earplugs can't really attenuate more than 32 dB or so across all frequencies). Unless you're particularly prone to damage, and are at a REALLY loud concert, you should be fine. But since this is new for you, you might try staying as far away from the loudspeakers as you can, and for the first time at least, only stay there for a little while. See how your ears react the next week or so, and based on how they are, go from there.
For me, it's important to make sure I protect what hearing I have left, but I still want to get out there and enjoy it while it's there. Everyone needs to find their own sweet spot.
Good luck!