What is a nominative case?
Nominative case is the case for the subject of the sentence - the thing that actually does something.
The nominative case is the case that the subject(s) of the sentence go in.
The subject of the sentence is the thing that is acting, e.g.
I am going to church
'I' is the subject, because it is 'I' that is actually doing something, therefore 'I' would be in the nominative case.
Nominative case is often talked about in conjunction with the accusative case, which is the object of the sentence, e.g.
you read this.
'You' is the subject because it is 'you' that is actually reading. 'This' is the object, because it is simply being read, not doing anything for itself. Therefore, 'you' is in the nominative case, and 'this' is in the accusative case.
'Nominative' comes from the Latin nomen meaning 'name', because it is often the first named thing.