[promethius9594]

---Does it automatically trap a killed creature if i use soultrap weap or spell?

assuming you have a soul gem large enough, that the creature is not a human (it must be an animal or a daedra) and that it doesnt resist the soultrap spell.

---Which gem will it go into? Do I have to select the gem?

it will go into the smallest soul gem which can contain the soul. dont soul trap an ogrim if all you have is petty soul gems and a grand soul gem... since the ogrim can't fit in a petty soul gem, it goes to the largest soul gem, the grand soul gem. thats a wasted soul gem. in fact, once you get to a certain level, you dont even need to trap anything other than golden saints,
until then, dont carry any grand soul gems you find, store them until you can kill a golden saint.

---Where can I buy yellow ones?

go to google.com and look up UESP Morrowind quest walkthrough. look around at some of the information (BUT BEWARE-- all the information to all the quests is there, so it can throw your gaming experience as well). the normal grand soul gems can be found there, however, they can also be found as random loot. i've found around thirty five of them so far, and have most of them filled with golden saints.

---What are the largest creatures that will fit? I mean what relationship does creature size and spell I want enchanted have???

all creatures CAN fit into a soul gem. it isnt based on size, but rather the strenght of the soul. even one of the demigods in the game can fit in the unique soul gem "azura's star"

---Can I choose any item for any spell? certain items are better than others for certain spells. look at the thread i made called "a beginners guide to enchanting" to learn some basics on items to create and some tips on how to make them.