A helpful video on YouTube with a basic introduction to the cases of Russian nouns. This is just one video lessons created by Максим Ачкасов on Russian as a Foreign Language. This video is only an introduction, and he goes into more detail in other videos.
The Russian grammar page at Wikipedia has a lot of information in general. Nominal declension is subject to six cases – nominative, genitive, dative, accusative, instrumental, and prepositional – in two numbers (singular and plural), and absolutely obeying grammatical gender (masculine, feminine, and neuter).