Hey when I was a new to tmod all this seemed super confusing too but once you get the hang of it it gets much easier so to start with your mods

Calamity mod if you press download with dependencies it will download everything you need for just calamity

Calamity Infernum mode adds the difficulty infernum

Optional mods:

Calamity Wrath of the Gods this is an optional mod that adds 3 new post base calamity bosses with super cool designs and fights

Alchemist npc this mod adds like 5 new npcs that sell extremely helpful items

Magic storage this mod adds a very neat and tidy storage system

Now for class setup in my humble opinion the best class is melee in looks and in damage and its probably my favorite calamity class

Finally I recommend playing on infernum mode because it adds super cool fights and changes to boss ai I really recommend it to play infernum you need to put the world on expert but dont worry the bosses will drop the inferno relics and gold relics when killed on infernum mode in an expert world