It is spelled 'till. An apostrophe is needed because it is part of a word- until, 'till.
It is " 'til' ", it comes from the word "until", which only has 1 letter L . Yes, you DO need an apostrophe.
It's both because 'til is shortened for "until" but if you look up till in the dictionary, you'll find that one definition is "up to the time of; until: to fight till death."
It's both because 'til has come into common usage over the last hundred years or so, as a contraction of until. However, till is not derived from until. Till is the older word, in use for about 1,200 years, whereas until is a relatively recent word, maybe 800 years old.
(tĭl) tr.v., tilled, till·ing, tills. To prepare (land) for the raising of crops, as by plowing and harrowing; cultivate. [Middle English tilen, from Old English tilian.]tillable till'a·ble...