Though, as beardmonk has recommended, you should really get started with a demo game, get acquainted with the system, and get to know your meta. As playing a glass cannon or brick force in warmahordes is completely different than the way 40k and fantasy has taught you. Also your meta can massively impact your army composition, especially if you want to play ret and realize that your meta has loads of anti-stealth units which will completely hurt you, if you choose ret of course.
At the end of the day, choose an army that appeals to you aesthetically first, then be prepared to lose a lot. Like...a lot. It happens to all of us. Good on you if you continue with this game. Though i do recommend hitting up privateerpressforums for more info. Battlecollege is good for quick references, but the information isn't regularly updated and it's extremely biased. At least in the privateer forums you'll have several different opinions, a lot of which come from actual tournament players whom can help guide you along.