So far, I've been spreading my passive skill points here and there in the huge passive tree. I'll probably want to change it to a more clever distribution later, when I'll have a better understanding of the game. Is it possible, and how ?
There are ways to make small changes to your skill point distributions in the passive skill tree, however the intention is not that you simply 'reset' the skill tree and instead make a new character if you want to make a large quantity of changes. This is detailed on the Path of Exile website:
You'll get a few respec points from quests which each allow you to remove a single skill point, so you can try out big nodes and undo little mistakes. This doesn't remove the need to plan your skill point distribution ahead and you'll still have to pay attention.
It is not possible to completely reset your passive skill tree.