If I have a ring with 15% resistance to fire and an amulet with 10% resistance to fire do they stack? Will I have 25% resistance to fire if I wear both? Also is a dragon's fire breath considered magic? Would resistance to magic protect against his breath?
Thanks

Yes, yes and yes. Having clothing and armor that is enchanted is a good way to build up resistances to what you are weak to. Dragon shouts are considered magic so magic resistance does indeed help protect you from them.

Though too much protection can ruin the challenge of the game. A Breton character can easily become immune to dragon attacks (except biting), so watch you don't "over enchant" yourself. Far better to have a lower immunity to a wider range of attacks than invincibility to one.