You don't need experience to start getting experience
you can get the whole transcription for book 1 and more
here (
although only the first 120 pages show up for me at the moment 
) if you don't want to work directly from the comic pages. If you're a bit savvy it shouldn't be too hard to import the CSV file into Excel or something*.
Be warned, though, it's a lot more time-consuming than it would seem. Also, it's really not that easy to do a good translation (especially of the literary kind, IMHO). But do not be discouraged, it's also really not too hard to do a translation that's good
enough. Just take your time, try to be aware of subtext/nuances, and make sure what you write in Portuguese sounds correct and natural. Don't hesitate to get back to your work after a break; in fact, do get back to what you've written after a break (and/or have someone else go over it), it will only improve the quality.
After you're done translating a few pages (or at the same time), you'll have the happy job of putting your text into the bubbles in an image editor... Unfortunately the wonderful Transcription/Translation tool linked above isn't able to upload files for an automated integration (yet).
*
if you're not, I should have at least the prologue tucked away somewhere
