If you want an event that allows you to equip a skill, you can do the following:
@>Conditional Branch: Actor[id] has item[id] equipped <--(if the specified actor/character has the item equipped)
@>Change Skills: Actor[id] + Skill[id] <--(add a skill to the actor)
@>Change Skills: Actor[id] - Skill[id] <--(otherwise, take that skill away from the actor)
There are a few disadvantages to using this kind of event system in the game.
1. The event will have to run as a parallel process on every map in the game. If you forget to add the event to a map, the player may find himself trying to swap her equipment and skills and nothing happens.
2. You will have to make a conditional event which corresponds to every combination of an actor and item giving skill. If you had 20 pieces of "materia" in your game and 8 different characters that could equip it, you would have to make 160 conditional branches. Not only does this give you a lot of room for error and make debugging difficult, it could potentially cause event lag.
3. You can only equip one "materia" at a time due to the equipment slot limitations of RMXP and RMVX. If you wanted to equip multiple "materia," you would have to have different kinds that fit into the various equipment lots, such as materia meant to go in the shield slot, materia for the head slot, etc.
If you are using RMVX, you can find a nifty script here:Assign Skills with Items
This removes the necessity of having to create all those conditional branches and put an event on every map. You can also create one use items that teach a character a skill. The disadvantage is you will have to define each item and the skill it teaches in the script, although it's a lot less work than creating conditional branches for each character and skill. If you had 20 items and 8 characters that could equip them, you would only need to modify the script 20 times for it to work.
Unfortunately, the above script is still restricted by the equipment slots of RMVX. There is a potential solution here:Custom Equipment Slot Script
but someone will have to translate it before we can take advantage of its awesome.