Either is used with a negative verb when you are agreeing with something someone doesn’t do or like etc.

For example:- B agrees with A in the negative

A – “I don’t like cheese.” B – “I don’t like it either.”
A- “I haven’t seen Lord of the Rings.” B – “I haven’t seen it either.”

As well / Too are used with an affirmative verb when you are agreeing with something someone does or likes etc.

For example:- B agrees with A in the positive

A – “I love ice cream.” B – “I love it too.” / “I love it as well.”
A- “I’ve seen Gladiator.” B – “I’ve seen it too.” / ” I’ve seen it as well.”