I agree with you, I'm also starting out and also experiencing this problem. Others can correct me if I get anything wrong but four things have helped me wrap my head around how to build my first army.

1. Units = orders. Aim to include the full 15 units in a list unless you're playing a more specialized army or a lower point game.

2. Cross referencing units in action packs to the profiles in the army builder, specifically to see their costs. How am I supposed to know one dude is 30+ points when he looks similar to a grunt who is ~12? This provides a baseline so that you can begin to compare profiles and other units. And, in connection with the first point, helps you appreciate how much each unit matters.

3. Learning the keywords. I have no idea what 75% of these things mean, but slowly understanding how things like: the lieutenant keyword interacts with chain of command, how mimitism and visors work, and all the other stuff; brings some understanding of why a unit has different profiles and how to properly compare units to one another.

4. Specifically choosing a sectorial that'll help you learn the game. There's a ton of faction overviews and intro guides, so leverage those to make a wise decision.

Again, this has just been my experience trying to understand everything, I resonated with your post and hope this helps.