The best, free resource is going to be
Archives of Nethys, which I've linked here to go to the on boarding introduction section. It is the free rules repository, officially sanctioned by Paizo. Highly useful in general. It's going to be your best source while Paizo is recovering from the hug of death the internet is giving their store.
The best introduction adventure is going to be the Beginner Box. The physical box is quite good, but so is the
Foundry VTT module. You will hear Foundry VTT recommended a
lot. The PF2e implementation is extremely powerful, automating many features like vision, flanking, resistances and weaknesses, and it has all the book content up to date.
The best adventure is generally considered to be Abomination Vaults, which is convenient because it takes place in the same town as the Beginner Box. Do not let the tagline "enter the megadungeon" fool you; it would not be the most popular 2e AP if it was merely a combat slog. The Foundry VTT version of it is also phenomenal.
Pathfinder has it's own version of DM's Guild, called Pathfinder Infinite. The real 3pp powerhouse is Roll for Combat though, whose main designer is Mark Seifter, one of the lead designers on 2e itself (you're probably familar with them by now lol). It is the closest you will get to first party quality so far.
Common pitfalls:
-Because +10 past a DC causes a critical in either direction, +/- 1's and 2s are both common and undervalued by new players. Causing a -2 to Will Saves with a skill might not sound very exciting, but in general that means a +20% impact for damaging abilities that target Will, not +10%, since it has converted successes to criticals at the same time it converts failures to successes. It also means auxiliary effects on criticals become more frequent, which can sometimes be devastating and change the whole nature of a fight. By the same token, let your defenses lag and you stand to get obliterated by frequent crits on your AC or Saves.
-Out of combat recovery is not only useful, it's expected. Spending the time to get focus points back and recovery HP is considered a normal part of the game. Multiple combats without any form of recovery are rare and highly difficult.
-The wealth per level guidelines, and the item system in general, is
not optional. It's part of the math of the game. The same goes with the encounter guidelines. It is not like most D&D editions where you can just ignore item and CR recommendations. Mark put a lot of effort to make the math of the game work when you just follow the guidelines, so I would tell any new 2e GM to consider those guidelines sacred until they understand why they are the way they are. All the APs already do that for you.
-Sometimes, characters with action efficient classes neglect to find a use for their third action. This is a mistake. Every player should have something to do with that third action, be that raise a shield, cast
shield, recall knowledge, or ready to Aid another player.