Watashitachi - means we, us.
Since in Japanese the word order is different than in English, to say 'we are' you use the following format:
Watashitachi wa (what you are) desu.
'wa' is a subject marker. (It directly follows the subject) 'desu' is a copula. (It links the explanation to the subject) Desu is usually translated into English as 'is' or 'are'.
There is no difference between 'is' and 'are' in Japanese as far as I can tell.
Watashitachi wa ureshii desu. - We are happy. Wareware wa kanashii desu. - We are sad.
If you wanted to say 'I am', you would use the same format.
Watashi wa ureshii desu. - I am happy. Watashi wa kanashii desu. - I am sad.
'Watashi' - I, me.
I hope that helps. ^^