Hi, i just recently had Brelyna follow me, but i want her to do more damage with her spells. Is there a way to make her equip new robes? because right now, she just won't.

http://www.uesp.net/wiki/Skyrim:Followers#Follower_Equipment explains it better than me.

It's all about armor rating. All Robes have 0 rating, so she will keep her old robes. Try equipping her with some Light Armor that you have enchanted. And give her a few staves. Followers love those.

If you want her to do more damage with spells, what you equip her with in terms of gear is irrelevant. There isn't any fortify school gear that increases damage, only the alchemical fortify school effect does that, and even if there was, it wouldn't work anyway because followers don't benefit from the fortify school magic effect.

TL:DR

Giving her stronger robes won't increase the damage of her spells and won't reduce the cost of her spells.

informative, thank you

Best thing to do with Followers is to armor them up and then give them a good weapon that compliments or contrasts with their preferred combat. In other words, Lydia loves to Melee, but I give her a couple of Staves with Elemental damage on them and she uses them at a distance. I once watched her Dual Wield a Fire and Ice staff, that was fun to watch.

I don't think there is anything that you can do to enhance their magic, other than items that reduce mana regen rates.