You don't learn spells normally. Instead, when you are hit by a spell, you may half the damage dealt to you, the health threshold to avoid its effects, or the duration of effects to absorb the spell. This is how you learn spells, this applies even if you aren't a spellcasting class.

If you are a non-spellcasting class, write down the spells you have absorbed and your spell slots somewhere, like in the notes section. You start with one spell slot of each spell level. Absorbing a spell you already have causes you to gain another spell slot of the spells level, if you absorb a cantrip you already know, roll a d8 to determine the level of the spell slot, if you roll an 8 roll a d4, on an even number, gain a 9th level spell slot, on odd, stay with 8. You do not gain spell slots equal in level to the slot used to cast the spell, only the spells original level.

If you take lightning, radiant, or fire damage that requires a die roll to determine the amount of damage you take, you can use your reaction to gain a number of "ability points" equal to the minimum amount of damage you could have taken from the roll, ignoring immunities, resietances, and vulnerabilities. You can use your ability points for a number of purposes. During a short or long rest, you may spend any amount of ability points you have. Each spent ability point permanently increases an ability score of your choice by 1, to a maximum of 30. You may cast a spell without using a spell slot or any components by spending an amount of points equal to the level you are casting it at. You may spend ability points to give you a +1 per ability point spent bonus to any damage roll, ability check, attack roll, or saving throw you make.

If you attune to an object, you may absorb it instead if you wish. This destroys the object, but you gain its magical properties. Any charges an item has are given to you and can be used to fuel any purpose requiring charges granted by attuning to or absorbing an item, you regain all charges, regardless of how the magic item worked, after a long rest. Additionally, you can absorb a weapon without unique magical properties, such as a +3 mace, or a nonmagical mace, to gain the ability to turn your apropriate body part into the item. This also applies to tools, instruments, and armor. Absorbing an item you have already absorbed gives you a +1 bonus to everything involving it. Rolls, AC, whatever applies. This includes charges and levels at which you can cast spells granted by items 

If you are missed (or succeed on a saving throw) by a unique ability, such as a dragons breath weapon,  you absorb the energy and gain the ability for yourself, and can use your magic item charges to use them. Aborbing an ability you already have causes its save DC/+ to hit, damage, and range.