Would like to know what kinds of authorizations and documents I would
need to do this kind of traveling...
You'll need only your passport and money.
I'm assuming you're a Brazilian citizen and will thus enjoy visa-free travel in the Schengen area for up to three months.
Please note, though, that you'll be unable to stay in some hotels, as they sometimes have age limits. But in most cities you'll be able to find a youth hostel or hotel that allows for 17 year old guests. When I was your age I did a fair amount of traveling in Europe, and it's not that hard. But you should definitely check with each establishment regarding your age prior to booking.
I do reccomend that you bring an approval letter from your parents, especially if you might be considering going to Croatia. More details here.
I'm not sure if this sort of authorization document would also help you with hotels. So, seek out places that allow minors even without such a letter.
Other than hotels, I can't think of any serious limitations for you as a 17 year old traveling in Europe. (There's not being able to drink alcohol in many places, and being unable to drive cars, obviously, but apart from that.)
Regarding your passport, take very good care of it and keep some copies (photocopies or digital) of it, in case something bad should happen. (Let's hope it doesn't, though.) For the same purpose you might also consider bringing a secondary ID card.
Be careful and have fun!