in the last lesson i did i saw some sentences like this "わたしもアメリカ人です" but why but a わたし if there is already a です? i'm sorry if i missed something in the lessons but i don't understand so please help.
I think it's the も that makes this sentence require わたし - アメリカ人です ("I am American") versus わたしもアメリカ人です ("I am also American").
And to break it down a little further ...
アメリカ人です "I am American"
アメリカ人 = American (literally "America Person")
です = copula, equivalent to the verb "to be" in English ("is/are/am")
Notice that the subject of the sentence is not stated in the Japanese sentence. It is implied by context. If nothing is otherwise implied, the default subject is assumed to be the speaker ( わたし ) when it is a statement, or the listener(あなた) if it is a question directed at someone else.
In the sentence わたしもアメリカ人です, you cannot leave out the subject, because you want to say that you are ALSO an American. The particle も is used to indicate that something ALSO applies to the marked noun. You can't use the particle by itself. It has to attached to a noun, so the subject has to be stated. It cannot be left out, in this case.
わたしもアメリカ人です "I am ALSO American"
I've observed, that it is used to insert another particle. Here it is the も which indicates, that somebody is doing/being something/someone additionally.
We're required to use わたし because of the particle も (which means "also" if you haven't figured it out already). Subject/topic particles (like は、が、も、 etc.) can't be used without their subject; もアメリカ人です is not correct.