Well you need first to talk to a Goblin named Mutamix in Thanalan after you reach level 20. Then you get the ability to convert all your gear that is 100% spiritbonded into materia. That's how you acquire it. The type of materia you get is depended the the ROLE of the gear it's made from.

So if you are converting a healers item, you get healer materia, caster, caster materia, tanking gear? Tanking materia.. and so on and so forth. Once you convert your equipment into materia it's gone forever. :O

To spiritbond an item you need to do things that are reasonably difficult and gain experience with it. So don't think at level 20 you can just slaughter level one mobs and get spiritbonded.