you use neither when none of them are correct and you use either when both can be correct. :)
2nd Answer:
"Either" goes with "or", as in, "Either you or I will go". That means one or the other.
"Neither" goes with "nor", as in, "Neither you nor I will go". That means that nobody will go.
either
either
No, it is not an adverb (except when it is used, informally, for the word "either"). It can be a pronoun, adjective, or conjunction.
15 squared is either a two dimensional area or simply a number. Neither of these can have a volume.
Too is a synonym of also. Either is accompanied by or in a sentence. This means that when you use either you mean one or the other, and when you use too you mean both. These words are not the same.