FINAL FANTASY VII
31 Achievements
1,000
25-30h
Fledgling Summoner
Use a summon Materia in battle
15
21.12%
How to unlock the Fledgling Summoner achievement in FINAL FANTASY VII - Definitive Guide
Your first chance is at the Chocobo farm.
After Leaving Midgar you have to go to Kalm to push the Story. Your next stop will be the Chocobo-Farm (Eath-SouthEath of Midgar). There are some Chocobos Behind Fences. Just "Talk" to them. They give you the "Choco/Mog" Summon Materia. Put it in one of your Slots go in a battle and use it. Done.
If you don't go to the Chocobo farm, the Shiva Summon Materia is nearly unmissable (except of a full Materia storage). In Junon you have to save Priscilla. She gave you Shiva as a reward for saving her.
Here are a list of all materias and the finding locations.
https://bit.ly/2HHYEYT
Note: The original link have () and did not work propper.
Edit: Added TheGreatKoala's alternate solution
There are multiple types of Materia in the game. To use a summon type, you will need to equip it into a weapon or armor slot just like other materia, and a new "Summon" option will be added to the command menu during battle. It will cost a specific amount of MP to use, and will bring forward the creature to assist you in battle. Use any summon materia to unlock the achievement. You will get many specific summons as part of other achievements, but any summon will work.
Simply equip a character with a Summon Materia, use that summon in battle, and you will obtain this trophy. The earliest opportunity to obtain a Summon Materia is at the Chocobo Farm. Interacting with the Chocobo by the fence will yield you the Chocobo/Mog Summon Materia.