Magic weapons, magic armor, and sometimes magical items have materia slots. Weapons have slots based on their relative size. Slots are essential to Materia Combos Magical weapons and armor have additional slots. +1 magical arms and armor have 1 extra slot. +3 weapons and armor have two additional slots, and +5 armor and weapons have 3 additional slots. So a +3 two handed weapon would have 5 slots.
The number of slots a magical weapon may have is based on weapon size, as seen below:
One handed: 2
Two Handed: 3
The number of slots magical armor has is equal to the armor bonus divided by 3. Therefore, +1 full plate would have 4 slots.
A light shield provides has 1 materia slot, a heavy shield allows 2 materia slots, and a tower shield has 3 materia slots. Therefore, a +1 heavy steel shield would have 3 materia slots.
Bucklers always provide only one materia slot, regardless of enhancement bonus.
Unlike Pathfinder, there is no spell failure chance with materia, so you can safely use armor while using materia. Of course, if you aren’t proficient in armor, you might want to take a feat for that.
A materia that isn’t in a magic item, a weapon, or in your armor isn’t really good in a fight. It won’t gain MXP (see Materia Growth) and can’t be used in combat. Even the most adroit materia users have to take about 15 minutes to slot materia, so it can’t be done in combat.
Slotting multiple materia of the same subset in the same weapon(two Ice Materia) is pointless, as neither materia will gain MXP or be able to be used.
Magic items will randomly have a slot, determined during item creation. There is always a 20% chance a magic item will have a slot, and a 5% chance that there is a materia slotted inside.