Is there a beginner's guide somewhere? Like a buying guide? Etc.?

You can read these three articles just put out recently.

Empire - https://www.fantasyflightgames.com/en/news/2016/9/2/you-will-pay-for-your-lack-of-vision/

Rebels - https://www.fantasyflightgames.com/en/news/2016/9/21/it-is-the-future-you-see-1/

Scum - https://www.fantasyflightgames.com/en/news/2016/10/3/as-you-wish/

They aren't perfect but can give you an idea of what the factions look like currently give or take and then you can get an idea of which one you want and build from there.

If you want scum, you'll definitely want to get the new Jabba stuff when it comes out

Boardwars has a decent guide all of the expansions and their opinions on how well they work in skirmish and campaign. Its not a bad place to start reading up on the packs. Other than that, ask around there are lots of opinions on all of the figures.

http://boardwars.eu/ally-and-villain-packs-for-star-wars-imperial-assault/

Step 1: Buy 2 cores and one of everything else.

Step 2: ________________.

Step 3: Win.

I'll be honest, I haven't figured out Step 2 yet.

Any reason to why you'd need 2 core sets? I've only been playing campaign but I've been thinking of getting into skirmish.

Coincidentally enough, if the above resources just aren't enough, the next episode of The Jodo Cast (episode 41) which should be out early next week is all about getting into Skirmish.

A good post popped up over on Reddit about starting skirmish. It addresses the current state of the game and is worth a read if you are still looking for more info. Just remember it is an opinion piece and that everyone does like different figures.

https://www.reddit.com/r/ImperialAssaultTMG/comments/561cft/beginners_guide_to_imperial_assault_skirmish_and/

The latest episode of The Jodo Cast is up and is all about getting started in Skirmish. (Also there's a giveaway in the episode.)