Here is my list of where I would start. You could play for years with just the stuff on the list.
Once you have the items on my list, I'd say the NPC Codex & NPC Codex Box would be very helpful, given the prevalence of humanoid enemies in your typical game. However, that might be less important since you have the tokens that came with the Beginner Box. Some people also really like the Combat Pad. Personally, I prefer index cards.
I suggest that you avoid supplementary mechanics at this point -- at least until you get a firm grip on the Core -- but given your stated preference for Sci-fi elements, you might want to look at the Technology Guide. Despite being part of the campaign setting line, it's the best resource for super-science and tech elements in Pathfinder.