Firstly, try to find an FC or static.

If you are genuinely worried about picking a job by it's raid viability, and everything looks interesting, you need to get into a group and fill what gap they need. Paladin will take you far, but any job you settle on will get you through everything if you work at it. Every static will use different party comps, so one group may need a bursty DPS while another is short a tank. It varies.

Otherwise, if choosing a main is impossible, then work a little on a job from each category. If you think RDM looks amazing, then that's your caster. SAM look great? That's your melee. What about a healer or ranger?

At least narrowing it down that way will help you shrink the pool of choices for later.