'Too' and 'also' have the same meaning.
'Too' usually goes at the end of a phrase. 'Also' can go in different positions.
'As well' has the same meaning, and usually goes at the end.
For example:
Ana speaks Portuguese. She speaks Spanish, too.
Ana speaks Portuguese. She also speaks Spanish.
Ana speaks Portuguese. She speaks Spanish, as well.
'Either' is different. You use 'either' in a negative sentence.
Ana doesn't speak speak Japanese. She doesn't speak Chinese either.
We often use 'either' in short negative answers:
A: 'I can't do this homework' B: 'I can't do it either. It's too difficult.'