Just a couple of notes from my experience to help with this process in case you are more familiar with the other 'greatest roleplaying game', I will wait to decide on what I play until after everyone else decides so I can fill in the gap :)
1. Casters are not single target damage dealers in this game anymore than the other game. They do excel at AOE damage, but mostly are bent toward buffing the party, debuffing the enemy (most of their spells have conditions applied even on a success, just to a lesser degree), and as stated before AOE damage.
2. Martials have so many options, but most of them are horizontally scaling as you level. What I mean by that is your feats are going to give you ways to do things to manipulate your action economy *your three actions a turn*, rather than just giving you bigger or more damaging options. For instance there are common feats for the Champion and Fighter to allow them to Raise their Shield as a free action on a turn, freeing up an action for a maneuver, second or third attack, or a skill feat like Demoralize.
3. On the subject, using your action economy to just swing three times in a turn is often not the best use of your actions, and investing in things like intimidate for demoralize or diplomacy for things like bon mot, lore skills for recall knowledge to find monsters weak saves and vulnerabilities, and acrobatics to tumble through to gain flanking are all great investments. Most classes are very much not MAD (multi attribute dependent) and you get four boosts ever time you can raise attributes, so you can usually invest in an alternate attribute than what would commonly be seen as 'normal' base on other systems. For instance, a Barbarians needs Strength, Con for Hit points, but they have no issues wearing armor, so don't also need to raise DEX that high because they get Medium armor proficiency up to master by later levels. So they have the free ability boosts to raise Charisma for intimidation checks.
4. Flanking, position, and mobility are all very real parts of the game that need to be taken into account. If you stand toe to toe with a monster, not utilizing flanking with your allies, and not moving out of the creatures attack range, then the monster can use all three actions to attack you the same way you do to it, and while they also face the MAP (multi attack penalty) for each subequent strike in a turn, the math is set in such a way that monsters of the same level as you will have a higher chance to hit on average, not by a lot, but enough on average to make striking and moving away so that the monster has to use an action on their turn to move to you a viable option. Obviously ranged combatants, and creatures with attacks of opportunity (Note: attack of opportunity is not something everyone can do, there is a feat you have to take to gain the privilege, and has to be listed in the monsters stat block for them to do) change this dynamic. But you can always use an action to 'step' away from an enemy, akin to disengaging, and then move away if in trouble.
5. Bonuses of the same type do not stack, but untyped bonuses can't be of the same type, they stack. Little additions go a long way to helping, so magic weapon, bards songs, bless, etc really do help even if they 'only' offer a +1.
6. Medicine is almost a required skill for someone to have, it provides a very reliable way (with healers tools, or the primal healing mud spell [can't remember the name]) to heal inside (battle medicine) and between combats. Pathfinder 2e's combat system, and Adventure Path designs, usually allow for the time necessary to heal everyone back up to full. Lay on Hands or other focus spell healing is also good for this because Focus Points can be replenished an unlimited number of times so long as you have 10 minutes to Refocus. Focus points only replenish to 1 focus point per 10 minutes (even if you have 2 or 3, and you can only ever get up to 3), unless you take a much later level feat that allows you regain more per refocus.
7. Multiclassing is done through the Archetype system. Whenever you gain a class feat you can instead choose an Archetype Dedication, so long as you meet the requirements. Once you have the dedication you can spend subsequent class feats to get additional abilities from that archetype. So you could for instance play a fighter who takes the Ranger Archetype, snag the Animal Companion feature, and throughout your journey raise that Animal Companion to be a matured and dangerous creature. You will have given up several class feats on that investment but it's about flexibility, and once again provides horizontal power scaling, not vertical. The system is very balanced, and there is no "You have to take this or your subpar" archetypes, feats, etc. Most every build will be viable in some ways. Tactics and Roleplaying trump builds.
8. Weapons and Armor will gain Runes as we level up, the two most common (and are expected by the math in the system) are potency and striking runes. Potency runes increase the attack of a weapon (just attack, unlike other games where a +1 weapon does +1 to attack and damage this rune only adds to attack). Striking runes add additional damage to the weapon, and scales with the type of rune (striking, major strike, greater striking), so that each level grants an additional die of damage. For instance a +1 Striking Greataxe will get +1 to attack and will do 2d12 damage on a hit. This is how the game makes up for you not sitting still in combat and swinging three times, instead making the one or two swings you do, do more damage. There are other runes as well that add effects to weapons and armor, things like flaming, frost, anarchic, etc. A weapon must have potency runes to add additional effects, and can only hold so many runes at a time. Runes can be transferred from weapons with time and gp. So if we find a +1 Mace, and you don't use maces, you can go and have that rune transferred to your weapon of choice.
9. Crafting is not a way to break the economy, think of it more as a way to have access to resources when not in a town. So you aren't going to make 10000 potions and sell them and become rich, you'll usually break even or maybe break even. However, if you are 4 levels down in a dungeon and your shield breaks after taking a massive hit during a shield block. The party crafter can help you fix that without having to go to town to find a blacksmith.
10. One of the most commonly debated rules in fact deals with Raise a Shield and Shield Block. Anyone can put a shield on their arm and raise it, giving them +1 (buckler) or +2 (most every other shield) to their AC. However, folks who have the Shield Block feat (granted free to many classes, and can also be picked up with a general proficiency feat) can use their reaction in a turn to Shield Block an attack. This is a form of Damage mitigation. A shield will have a hardness and a set of hitpoints. The hardness is a Damage Resistance, so if the hardness of the shield is 5 for example, and an attack comes in for 4 hitpoints of damage, and you shield block it then you take no damage, the shield takes no damage, because the attack didn't penetrate the hardness of the shield. However, if the attack exceeds the hardness of the shield then both you and the shield take the remaining damage. So an attack of 8 damage would be reduced by 5 due to the hardness of the shield, and then you and the shield would take 3 damage. The shield will also have a BT stat, this is it's broken threshold. If the hit points of the shield dip below that BT but not so much as to deplete it's hit points entirely, then it is broken but can be repaired. It is however unusable until it is repaired. If the hit points are depleted entirely then the item is destroyed. You get to decide if you want to shield block after the damage is determined on an attack.
