I would say your best bet is to try them all, at least to 50. You can solo all jobs now (yes some are easier than others) if you need to.

Playing all jobs not only gives you all available sub jobs but it also teaches you about the other jobs so when you party with them you will know what they can and cant do. A good example of this is THF. Anyone who plays thf knows how frustrating it can be to try to get off a sneak attack without the person moving and ruining it. Someone who has played THF knows whats going on and can help the THF in your party get his sneak attack off without moving out the way at the last second.

And by the time you hit 50 you have seen a lot of the world, gained a great deal of experience playing the game and know if the job is right for you.

I personally started off playing whm, then moved to smn, and found my favorite job in dnc. Still I have leveled each job and found things i like/dislike. If you want to find the job that's good for you just start with a job, play it and if you like it great, if not pick something else until you do. Don't be afraid to level something you would like to do based off other peoples opinions. It might also be a good idea to check out the job list here http://ffxi.gamerescape.com/wiki/Category:Jobs read what the jobs do, look at their spells/abilities and see if anything interests you.