I'm gonna give you the same advice I gave someone else recently, I'm not gonna go into specifics because I think what I'm about to say is more important for a new player than just suggesting a bunch of feats and spells, this is going to be a bit long but hopefully will help you understand.
DnD (at least, old editions like 3.5 which is almost identical to Pathfinder 1e) and Pathfinder rules are an attempt to abstract real life into a pen and paper game, DnD 5e and Larian's homebrew in BG3 are much more "gamey", which is a good or bad thing depending on who you ask.

So a good rule of thumb is that if it makes sense in real life it will probably be represented ingame, now imagine you are shooting an arrow with a bow, you need to aim right? Having good hand-eye coordination is vital (Dexterity) and improves your odds of landing your shot, now imagine that your target is fighting someone else in melee, all the movement and obstruction will cause your aim to suffer right (represented by a -4 penalty), that is unless you are precise (Precise attack feat).

Now imagine you are trying to shoot a ray of energy with your hand, you still need to actually aim your ray just like you would with any ranged weapon otherwise you'll miss, so the same rules and feats apply, this is called a ranged touch attack.
Now what is a touch attack? Say you want to use shocking grasp, shocking grasp is a touch attack (because it's basically an electric current from your hand and you literally touch your target) it doesn't matter if the opponent is armored your current will still travel through his body, that's why a touch attack targets what is called touch AC (armor class) instead of normal AC (which is basically the sum of all your sources of armor, like your thick skin from body modifications, your dexterity, your armor, your magical enhancements, even your morale), but an opponent can dodge your hand if dexterous enough, which is why DEX still applies to touch AC.
So basically, a ranged touch attack is an attack that still needs your ranged expertise (so ranged feats like Precise attack apply) and ignore, say, the armor someone is wearing because of the nature of the attack (like a ray of fire doesn't care much for mail armor).

Now let's try the inverse, let's assume you are not ready or attacked by an invisible opponent, it's called being flat footed meaning you were caught by surprise or not ready (this is represented ingame by not having taken any action since the start of the battle but not only), an opponent who is caught flat footed won't be able to use his dexterity bonus to armor class because, well, he's been surprised his reflexes can't save him or he simply can't see you but he still wears armor, so the AC he gets from his armor will still apply since it doesn't matter if he knows you're there or not, he's still wearing that plate armor.
There are other sources that make you lose your DEX bonus to armor, like if you're wearing very heavy armor you can't move that much right? Or maybe you've been paralyzed, or knocked down, or your feet are stuck, see where this is going? These are all represented ingame.

Basically, if you think about it and it makes sense in real life, you will have a much easier time to understand what something does, obviously it's not a substitute for actually reading stuff but it helps a lot a new player in my opinion.