When you look at your player bar you see your portrait, next to it your healthbar and ragebar. I'm assuming you're using Vicious Slash, which require rage. To build up rage you need (for example, later in the game you'll get others) to use abilities like Assault (which builds up 2 rage each time). So go like this: Use Assault to build up rage, use it like 3 times to build up 6 rage. Once you get to 6 rage, you can use Vicious Slash 2 times (since one use of VS requires 3 rage). Now don't stick to this rotation, was purely ment as an example. Hope this helped, Cheers