'Also', 'too', and 'either' are used to say the same thing but they are used differently. They are used to say that things or people are the same.
'Also' and 'too' are used in positive sentences while 'either' is used in negative sentences.
• ALSO
You can use it right after the verb "to be":
- Mary is eating. Bill is also eating.
You can use it right before an action verb:
- He likes cookies. I also like cookies.
You can use it right after modals:
- She will drink water. I will also drink water.
• TOO
You need to use it at the end of a sentence:
- He is listening to music. I am listening to music too.
• EITHER
You need to use it at the end of a sentence:
- I am not going to the party. Julia is not going to the party either.
WARNING:
Either can be used to mean something else and it will not be used the same way:
- We will either study maths or English.
Here, either is used to give a choice or an alternative.