For anybody starting hockey after the age of like 6, the single biggest hurdle to overcome is skating. People who have been skating since kindergarten age almost always have an advantage. So if you're just starting out, I would work on skating as much as possible (including taking any power skating lessons or classes available in your area). I've seen guys at pickup hockey who, during warmups, are doing all these insane looking dangle moves (while standing still) and you're like, "Oh great, one of these guys..." And then when the puck drops, they all of a sudden look like they've never played before, because they can't skate very well.
I don't know anything about the top amateur leagues in Finland, but if you've already got experience in other sports, are reasonably athletic, and work really hard on your skating, my hunch is "making it" to one of these leagues is at least theoretically possible within a few years.