This page is gonna give you some tips to recommend which class to play, which class is more fun or satisfying and some things you should pay attention to.
First, it is suggested to unlock them (Dark Knight, Astrologian and Machinist) all and take some runs through Palace of the Dead. Palace of the Dead will give you the chance to get a more realistic feel for the class that you'll be playing primarily at 60. Additionally, if you bring them into Palace of the Dead, they'll level up a bit too. It's a win-win.
Second, if you have only summoner and black mage don't go healer or tank with 0 experience in that role, you should level conjurer at least to 30 before picking astro, or gladiator for dark knight. if you don't wanna do that go machinist. AST is fun, though you'll need Conjurer cross-class for protect & stoneskin. On top of that, AST is probably the most finicky out of the 3 healers imo but it's definitely more fun when you get the hang of it. The thing is that, playstyle differs a lot from player to player. You should probably consider cross class skills as well. Both AST and DRK has a bit more need of those than MCH does.
Last is about the three jobs separately to let you figure out the differences. MCH will be dps so a bit familiar, but it's not really similar to the SMN playlist of not having a real set rotation and instead running through your skills on a priority system, activated priority skills when they are up. Wildfire is the only exception where you need to make sure everything part of your highest dps rotation is up for wildfire. Asto is neat if you want a healer with pretty unique play-style. They are pretty powerful and offer some great buffs, with a bit of RNG. DRK is a pretty fun tank. Gets a pretty high skill floor and skill ceiling too at later levels. Just run it through a few mobs and read the tooltips to understand what everything does first, maybe run some low level dungeons (instant queue times usually.) Playing Gladiator a bit could also help. You'll need to level it for provoke if you even want to do serious tanking anyway (though dungeons are fine without it.)
If you are troubled with the choice of the jobs, hope the above mentioned tips can solve your trouble. Any requirement for FFXIV gil can be found here. Wish you progress in the game.