You can complete daily quests in Arcane River (speak with NPC Rona), and complete the Erda Spectrum (speak with NPC Nina) to obtain Vanishing Journey Arcane Symbols, which can be double-clicked in your inventory to fuse with your current Arcane Symbol. Arcane Symbols also (very rarely) drop from mobs in the area, and are given out during events.
Each Arcane River area has its own symbol, and its own dailies for obtaining more symbols.
After you fuse a few symbols, you can enhance the equipped symbol by paying a certain amount of mesos in your equip inventory (you can view the meso amounts for each level here). Each additional level gives +10 Arcane Force and +100 additional main stat (+1750 HP for Demon Avenger, +39 STR/DEX/LUK for Xenon).
You can also use hyper stat points to increase your Arcane Force, although you shouldn't need it just yet.