I bought a key from Amazon and gave that to my friend. Then he went and made an account on his own and entered the key on there.
If you buy from Amazon, make sure you're buying the online game code, and not the physical copy. Unless you'd prefer to give someone the physical copy.
http://www.amazon.com/Elder-Scrolls-Online-Game-Code/dp/B00I4BGNVM/ref=dsvrt_myd_asin_block
The pre-paid cards are for those who already own the game. You could buy a digital download of the game to speed things up. Once your account is made/the game downloaded, put in your pre-paid card code and enjoy.