Having gone through The Horus Heresy books to look at the Legion rules, there is a lot to read and a lot to absorb - again very daunting!
So while surfing the net to find a definitive answer to how my War Hounds Legion should be armed and structured, I found probably the best written and straight forward article on making a 30k Legion. The article is from a really unlikely source too: 1d4chan! I'm not really an 1d4chan guy, but do yourself a favour and have a look over the way this article is written. It goes through each unit, the best things to include, or not, and why. It takes into consideration points, play value and in really plain straight forward language - it's so straight forward to get to grips with!
I found this SO handy and frankly having looked around the internet and through those big glossy pages of the Horus Heresy books, I would be floundering still if not for it.
I ended up using the two in conjunction with each other and had a blast! I've now divided the miniatures I had painted up into their new squads and worked out what I need to paint AND started applying squad decals to the right miniatures!
Hope this helps design your force and allow you to build your army quicker and with purpose.