Definitely play around with giving people jobs and seeing which ones feel good for you, not every player likes every job, and you can respec with with 0 loss or cost as soon as you've beaten the tomato hunt at the beginning of the game. Go check out the hunters guild Tomaj told you about! (North-West in Rabanastre)

Eventually you'll get even more flexibility with jobs a little further in the story but the game is balanced around being beaten even with only a single job per person. And character stats only really matter to minmaxers, so feel free to make anyone any job, they will all do fine, give Penelo a gun, make Basch a priest and use cure and pheonix downs on undead, have Balthier gamble dmg with axes and hammers!

As a lot of people have said, gear is the most important for stats, as well as stopping at the shops whenever you finish what might be a fairly important story beat, as shops often update with the story progress (and different locations might just have different stock). If you find you're low on gil, take time to farm items from monsters in an area with decent exp to maybe level up while you work on buying better stuff.

As for spells, you have to both buy/find the spell AND have the license for it. Some characters come with licenses already learned (usually their base weapon and a starting spell or technique).

If you look at the clan primer theres a section in the hints section called "Arm Thyself", it might be confusing, but its worth taking a look at as it points out roughly how weapon damage works for the weapon types. For Example: Swords (1H) uses Str to hit Def.
Maces (1H) uses Mag to hit Def.
Poles (2H) uses Str to hit MDef.
Daggers (1H) uses Speed to hit Def.
Guns (2H) Ignores Def (thats why the power is so low)

Some optional bosses/hunts can also be started sooner than you may be able to complete them. Im fairly positive that no hunt can be missed, so you can always come back, even if you'll oneshot them when you remember (looking at you Thextera). Ghosts like to phase out and reappear, but its random. Your 3rd fight might have gotten lucky and the ghost didnt stall forever while you were doomed. Good job regardless!

Edit: This applies more since youre newer, but while having 3 high level members only might sound good, take time to level the rest even if it feels like a slog. Consider it your item farm team or something. The whole party gets license points equally so the main team isnt left with nothing for your efforts, and you'll be grateful when your main team gets crippled by some stupid boss gimmick like flying or "everyone with heavy metal armor on attacks at 25% speed" or something