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:
If you want to adjust your passive build during gameplay, there are Passive Skill Refund points available from quests or relatively rare items in the game that can be found or traded for. Fixing mistakes with a character build or improving small-medium sized aspects is relatively easy, but our intention is that players who want to try substantially divergent character builds are encouraged to play a new character through the game, organically leveling it up rather than just respeccing into it.
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.