Lo mejor que puedes hacer ahora es recordar las frases completas a diferencia de palabras distintas, en este caso no tendrás ningún problema con elegir los nombres según el sentido de la frase que quieras usar

Hi.I can help you. Please, tell me what do you don't understand. Do you want to know about nouns in general?

Clarify what?
Meanings or forms?

The first is rather easy, our cases (or sometimes preposition+case combinations) do mostly the same job as English prpositions (sometimes they dont' match perfectly, that's where you will make mistakes).
There must be some table somewhere that sums it up neatly.

The latter is scarier. I don't know how to facilitate memorization of those forms.