Can I sell my game freely, or do I have to pay licence to sell my game on Steam etc. ?
If you have the Unity free version, you can sell your game on Steam for PC platform without purchasing license. If however you make more than $100 000 in sales for your game, you are expected to (and should want to :p) purchase the Pro version of Unity.
For selling on other platforms, such as IPhone etc, you need the relevant license to publish Unity to those platforms.