You'll join the grand company at level 20 after completing a certain quest of the main storyline, after joining one of the grand companies(you'll get to know them ingame through a brief speech) you will be able to get company seals in order to exchange for items and rank up your company rank in order to get even more items
Those seals can be obtained in several ways
1. Grand company leves - those aren't different from your usual leves, except they are given by grand company officials.
2. Supply missions - these ones ask for certain materials or crafts of each craft/gathering class(related directly to your level) and given exp/seal rewards that scale accordingly.
3. FATEs - probably the best way currently, finishing FATEs after joining a grand company awards you seals and unlike company leves or supply missions, those are not limited by allowances or timers, only by your desire to beat the living hell out of the poor FATE critters.
4. Expert delivery missions - Unlocked at the sergeant rank, this allows you to exchange green/pink items you don't need/want for somewhat small amounts of seals, not the best, but since its for stuff you don't need, its fine.
p.s. Chocobo license does indeed cost 2000 seals but that should be relatively easy to get.
Edited, Aug 31st 2013 1:54pm by MitArgento
Edited, Aug 31st 2013 1:55pm by MitArgento