pick the one you think looks the coolest. if you level up and eventually find out you dont like the class playstyle, simply pick another one to try. You don't "waste time" by leveling a character and then abandoning it; eventually you'll want to have a large number of leveled characters.
#Newbie need help
sharp socket
but if I had to give a recommendation, I usually point new players to any Explorer class
twilit prism
All classes have some form of bossing and mobbing. I’d think of it instead as like, do you want a more straightforward experience or more skill/ combo playstyle, do you want to manage summons ( everyone has summons in the 5th and 6th job, so I’m thinking in addition to erda shower and sol Janus)or do the attacking yourself? Some classes have lots of active skills, or some have mostly passives with a few important attacks. Then of course, as hexa mentioned, what fantasy/weapon group do you find cool/interesting