Dark Alliance

Firstly, how to get a parry is "It's All About Timing". To do this you want to have an enemy attacking you and JUST before their attack would land on you, you should tap or press and this will not only block the attack but also stun your opponent.

Not all enemies can be parried though. Good enemies for this would be the cultists. (The guys in robes that float and have double daggers) They're good for this because their attacks are very predictable and they attack often.

An example as to where you may find these are in the 4th main area of the story (The order of the one light) focused around the cultists or at the earliest you will find them towards the end of your first mission after the tutorial (Act 1 of Companions of Icewind Dale - Goblins at the gate). You will find them surrounding a cage, it'll be easiest to accomplish this by taking the enemies down to only 1 or 2 cultists so that it's easier to engage without becoming overwhelmed. Now just get to timing your blocks and good luck!

Edit: According to one of the commenters, TAB is your block/parry button if you're on pc :)

Edit 2: A lot of people keep asking me to mention the tutorial. This is a valid option if you want it to take a LOT longer unless you just can't seem to hit your parries at all. You don't get to parry as often although it is a lot safer. This is not the method I use or recommend unless you can't parry well.