We've all seen the film 300, right?
300 brave soldiers against an army of over 100,000.
Imagine that scene... You got it? Ok, now convert it into music.
To be frank, that's what you're getting with Viking metal - its brilliant.
If you're new to screaming and seem to be getting pu off by it then just try and focus on the musical aspect, you will adjust to the vocals.
A good starting band for this genre is Turisas. They're a great band with great sound quality (which makes all the difference when you're listening to a new type of song).
Recommended songs by Turisas:
To holmgard and beyond
The portage to the unknown