What is a good way to start my SciFi/Fantasy Novel?

Start with an action sequence of some sort. People will eventually be willing to wade through the most tedious of history lessons about imaginary worlds, but only after you've captured their attention and provided them with characters they might care about. From the brief sketch of an idea you've given here, which lacks the actual characters, that leaves you with two options.

The first would be a scene depicting an encounter with the raiders, a scene which explains(shows, not tells) the motivation of your tribe. The second would be a scene depicting an interaction with the civilization, a scene which forebodes the problems which will be faced by the tribe. Keep in mind this idea is based on you basically saying Group A, running from Group B, will have to deal with group C.

So obviously, the actions proposed are going to be A vs. B or C. There are hundreds of different scenes which could be written here, depending on what type of book you're writing. Now Robert Adams, who's popularity peaked sometime in the 1970s/1980s, would have started with a scene in which his horse nomads raided a settlement or in which the decadent civilization took advantage of the noble horseclansmen.

People would die, things would burn, and he'd likely grind his axe-pen over the evilness of the bad guys, by showing they were mutant-hating, homosexual, organic-food eating, Christians. Then he'd cut to some character building where we get to know the a captive girl who actually has super powers through a detailed description of her daily life in a nomad encampment. Those were his strengths, battle scenes, fight scenes, and well-researched depictions of nomadic life.

What are your strengths? Much of writing is writing what you are good at and learning to avoid things which you simply can't write.

Often even a book about huge fictional-historical events like what you're describing starts with something exciting or mysterious happening to one single character. This can grab a reader's attention more than a description of a huge but faceless battle or migration.

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.