really depends, but ill try to give u a quick run down on each job
warrior = most common choice, well balanced, 1 of best for sub jobs, easy to get money with, and fun to use
monk = very strong, but slow, can take a hit because of somewhjat higher Vit then war, and alot of HP
blackmage = very fun to use because u can nuke things, but very hard to lvl up because of constant resting needed to restore MP and HP, later on at higher lvl's its considered 1 of the most deadliest jobs in the game
whitemage = fun, but has its flaws, use it if u wanna help people like i do, very hard to get gil "trust me it is" but it is considered a very respectful job and u will make alot of friends fast, and lvl up faster do to people needing them
thief = attacks fast, very deadly at higher lvl's and with the job alility u can get rare treasure easier
redmage = how do i say this...its pretty much all the above jobs put in to 1, it can equip alot of the equipment out, cast blm, and whm spell's and they have of there own spells, this is also a good pick because of there wide range of uses
if theres anything else, or u want more detail just send me a msg, till then ima go to bed, good night!
If you're worried about picking the wrong job or something, don't be.You can make as many characters as you want, and as long as you keep just one at the end of the trial, you wont be charged for the other ones. Try out all the classes, and expiriment. The free trial is a perfect time for this because it's, well, free.