Full +15 armor will get you around 200 dp which is enough until you start farming in the desert.

The biggest increase for AP is your main hand weapon. +15 is not the cap, because after 15 it goes to PRI, then DUO, TRI, TET, PEN, effectively +20 in other words, although Tet and Pen are out of the question since you are so early on anyways.

You have to get Sharp Black crystal shards to upgrade from +15 to pri, duo, tri etc. You get them from any gathering task or marketplace. Going from 15 to PRI is 8 AP, DUO 8 more AP, and TRI is 12 extra AP being the best and safest place to sit at with your weapon.