From 0 to 20 you level quickly, so gear is not used long (or terribly important). A good weapon is useful and can be bought from the sellers in the adventurer's guild.

You can sometimes find the low level items for sale in wards besides Ul'dah (but not very often).

A fair number of low-mid level equipment comes from Leves (as rewards, or in chests you find when out and about).

I would not worry too much about good gear until 25-30 (aside from weapon). If you do a lot of leves until then you should have a decent amount of gil (I usually have about 500-600k from doing levequests till level 30).

This will then be plenty to get you equiped from the Ul'Dah wards until level 50 (provided you are fine with 'decent' rather than top of the line equipment).


Note:
Hopefully you did not choose Archer as your first (as arrows can get expensive). Or a tank job (as you may benefit more from armour). But even if you did, you still can manage, just need to be more frugal with your gil.

Also: The main story line quests give lots of good gil and XP, so do not avoid those ^^