Well, if I remember, spellcasting is based on fatigue, and I thought it was indirectly affected by your armor.
Eh, don't remember that much. Spell failure
is related to fatigue, but you have to actively go out of your way to drain fatigue (running, melee attacks, etc.) so it's not really an issue. Plus you eventually can make good, long lasting, fatigue regen spells and basically make it a moot point.* Or enchant some fatigue regen items... pretty sure you could do that in morrowind, anyway...
*Particularly if you're running a magicka regen mod. Which I'd suggest. It really makes the whole casting experience less painful. It's not that it makes it any easier, per se -- alchemy can break resource issues with little trouble in either system -- it's just that it removes a layer of inventory fiddling and makes the whole experience much more fluid.
Okay, I know I could probably find this out by reading the manual, but since you guys are so quick and so knowledgeable... How do I use the spellcasting slot? What key is it? And how do I add the amulet to it?
There's... a key to press! I've forgotten which. I usually rebound it to... something. Vaguely remember pressing r or f a lot, though there's some bleed through with oblivion (or nehrim, anyway) going on with that. There
should be a key config option thing in the options, I think. Think you can also add them to a hotkey list of some sort? 1-X, where X is a number I've forgotten.
Oh, and how do I learn spells? It couldn't hurt to know one or two, even if I don't have the skill to succeed in them - like Remuthra said, even if I fail at casting Mark, I don't need the magicka for anything else.
You buy 'em. Mage guilds are the likeliest place to find folks that'll sell 'em (and most (all?) of them also have someone that can make custom spells for you**), but you can find 'em in other places too. The shop-dude in the first town you run into has a few. Just look for "spells" in the interaction menu-whatsit.
**Though you have to know individual spell effects before you can use them in a custom spell. No making fireballs with customized parameters (in regards to damage, aoe, etc.) if you don't know the fire effect, no tailored healing spells if you don't know a restore health spell, etc.