Hi and welcome to the game, you can start wearing D-grade armor and weapons at lvl20. Most of them are sold in shops, but Top Grade Armor will only be found in private shops (people sitting on the floor)
C grade = 40+
B grade = 52+
A grade = 61+
S grade = 74+
You get D grade Expertise at level 20, but how much adena you have determines what you can afford at the time. And once you get to the mainland you can check out the NPC shops or if your lucky a player shop
You can start using D grades at level 20. Easiest way to get them is to buy from NPC shops in towns - but if you look around town squares you can sometimes find them cheaper than NPC.