As an adverb: After a negative statement used as an intensive meaning something like ‘likewise’ or ‘also’: ‘if you don’t order dessert I won’t either.’
As conjunction: Used before two (or more) alternatives: ‘you either passed or failed your test.’
The position of either should be the same as the position of or (i.e. immediately before a subject, immediately before a main verb or after a verb etc.): ‘Either stay or go.’ ‘You should either stay or go.’ ‘You should stay either here or at home.’