Few things you should notice. Polish has the instrumental case, you use it with verb "Być" when you want to say something is something, or you are someone ect. One object stays in nominative (normal) form and the second object gets changed. This changed form is created by: if something is female or just simply end on "-a", you simply change/add "-ą", książka (book) -> książką, mysz (mouse) -> myszą. So you can say "Jestem mężczyzną". Otherwise, you change/add "-em" (some nouns have different base form for cases) Kot (cat) -> Kotem, Drzewo (Tree) -> Drzewem, Słońce (Sun) -> Słońcem. So you can say "Jestem studentem". Remember that ż and ź are 2 different letters. The country is Polska, language is Polski (I speak Polish - >"Mowię po Polsku"), similary German language is Niemiecki. Notice that we use preposition "po" when we say that we speek given language "Mowić po polsku", "Mowić po niemiecku". and American person is Amerykanin.

Also you do that have to say "ja" before jestem jestem means I am therefore saying ja is just irrelevant