pre scriptum:
-i'm not too well versed in english, so i'm sorry for eventual errors
-if you don't agree with me -10k aura/s
-this is my opinion, i'll say this now so i don't have to write imo after every sentence from now on
First step: look out for what your party needs, every strong party comp needs at least:
-a dedicated healer and multiple diffused healing options + out of combat healing sources capable of fully healing the party in at least half an hour
-two frontliners
-one competitive font of single target dpr and two secondary damage dealers
-about those secondary damage dealers, area blasting and ranged damage dealers
should be present
-a caster, literally any caster will suffice.
-math adjusters (buff, debuffs)
-good recall knowledge and good skill coverage (at least one master in every relevant skill such as: diplomacy, deception, nature, arcana/occultism, religion, stealth, thievery), most of those skills can be aided through magical means but cannot be fully replaced until high level spells come into play
ps: unconventional party comps do work, but you need a good reason and a good plan behind them, playing them straightforwardly won't work
Second step: now that you found what role you need/want to play let's do a general rundown of what each role has to do.
(to clarify, characters can cover multiple roles but not equally well, it's strongly advised to choose something which is capable to be top tier in your "main" chosen role)
Healer.
a main healer is anyone who's capable of spamming the heal spell, it's probably the best spell in the game and the highest/most reliable source of healing. (chirurgeon is an exception to the rule, by my personal calculation it doesn't seem capable of fulfilling the main healer/only healer role, but i admit ignorance in this case)
-secondary healing option include things such as battle medicine, lay on hands, water kineticist things and generally every source of non spell-slot healing, it doesn't match a cleric but it sure as hell fills the gap and frees the healer's action to do something proactive in the fight and to not just react
Frontliner.
Anyone capable of handling themselves in the frontline, no one except a high level kineticist, a shield champion and a barbarian is capable of holding their own alone in the frontlines so every frontliner needs a buddy.
a good frontliner needs:
-a high/good AC + a way to boost their own AC and get that classic +2 circumstance
-a good amount of hps (at least 10 total hps per level)
-threat either through damage or control (for most frontliners: maneuvers)
-some way to reduce damage, the most common way to do so is shield blocking but resistances etc are also very strong
ps: rogue is kind of an exception to the rule, it offers basically nothing tanking wise but it has such an enormous threat and out of combat utility that it works perfectly anyway
striker/damage dealer
Their work is to output the most damage while keeping themselves alive to do so.
a striker's main job is to do damage, and to reach said objective dpr calculation is a useful mean; but a striker who only does damage is poorly effective.
ranged striker's need to focus completely on either damage and their other utility since they don't contribute with their hps, functionally reducing the party's hp's reserve
melee strikers need to optimize so that they're able to survive without a cleric pocketing them, this also applies to actual play through careful positioning
area damage, usually outputted by casters, can't really be optimized and therefore you should just try to not target high saves/resistances
Casters:
this is a far too broad point to discuss (i'm sleepy)
Math adjusters
This is something that everyone can and should do, anyone should try to apply debuffs and buffs in any given situation, the easiest and most common situation is "off-guard + -1 status" paired with a general "+1 status".
Third step: choose your class and make the character with your chosen role in mind
i can't give tips for every single class and tip, but i can suggest to look up various guides online which are usually very helpful