There are a lot of other resources out there, but here's a crash course. At an early level, you have the opportunity to receive a quest from Koffie to obtain a mag. Once you have your mag, feed it items to grow its stats. Make sure to equip your mag. As you feed it, its level will increase. Once it gets to level 30, it will evolve for the first time. It was also have a Photon Blast. Equip the Photon Blast to one of the hot keys. As you're fighting enemies and taking damage, this will charge up the PB. Once it's fully charged, it will make a sound effect and won't be greyed out. Press L3 to activate your PB and pop the trophy.
For greater details about every step and to learn about your mag, check out these resources:http://pso2.cirnopedia.info/mag_2.phphttp://www.bumped.org/psublog/pso2-jp-introduction-to-mags/
If you've read this far, please be cognizant that, like with everything in this game, you should do some research first; because, if you add stats to your mag you don't want or do it on accident, it will cost you real money to revert or buy a new mag. So, for your own sake, plan out what you want from your mag and save yourself some trouble. Most people do pure striking mags. So if you want to play a ranged character, only put points toward RATK.