The Hooded One
Very early in Act 1, you'll revive the Hooded One in your quest with Una. By speaking with him, you'll be able to refund your Passive Points on the Skill Tree.
Gold Cost
Refunding your Passives will cost Gold, and it shouldn't be too hefty to start out. However, as you get higher and higher level, the Gold cost will increase as well.
If you want to spend points early for quick power and refund it quickly, the Gold price shouldn't be too much.
Just keep in mind that later in the game, completely respeccing your Passive Tree will cost much more gold.
We don't know exactly how much Gold it will cost just yet, but it shouldn't be too bad after running some Maps.