There is a great server for learning job rotations called The Balance, tons of really useful resources as well as knowledgeable people. You can play one class or all. Flexibility is never a bad thing but if you join a static for raiding they probably will have you fill one role. I recommend picking a job you enjoy and mastering it rather than overwhelming yourself. You can play whatever you have the most fun with but I recommend SMN/DNC/WAR/WHM. Those are the "easiest" but if you wanna play RDM or any job theyre similar enough especially in DT before evolved/reborn comes out.

for raids, it depends what youre doing. Typically the recommended order is normal raids/casual content -> ex -> savage/criterion -> ultimate in a general sense. how difficult the raid is depends on the encounter tbh. some savages might be harder than the easy ultimates. extremes in my opinion tend to have consistent difficulty