new to series- i'm up to the second part of the quest for caius (trying to get the skull for the documents). i already found the skull but sold it (unaware that i needed it) and i can't remember who to. is there a way to get it back without having to completely start over?

Which game?

(This subforum covers the entire series; you should specify which game in the series you are talking about.)

"Caius" probably refers to Caius Cosades, one of the main quest-giving NPCs in Morrowind.

yes in morrowind. do i have to start over?

You should be able to buy it back from whomever you sold it to. If you don't have enough money, you can always insult the merchant until he starts fighting you, you kill him and after that you should get your items back (and all his merchandise as a bonus).

You could also use the Construction Set and cheat the item back into your inventory - but Sarisio's methods are certainly nicer and in line with the idea of role-playing: Buying the item back fits most roles, I guess, but stealing or killing is more for thieves, assassins and the like. :-).

You could also use the in-built console of the game:

From the UESP wiki:
"The console is accessed by pressing the key to the left of the "1" key: on standard American English language keyboards this is the tilde (~) key. On British English keyboards, this is the backtick (`) key."

Enter:
player->additem, "misc_Skull_Llevule", 1

This will place the skull in the inventory of your character.