First of all you have to start from something everybody starts from - alphabet. Then prononcuation (just repeat after native speaker - that's the part where you need a teacher or at least a native speaker, because you may not realise your mistakes (for example У and Ю sound). Then you study pronouns, verb conjagations and only after that - cases. It's the hard part :) I recommend to learn them one by one in the following order : Nominative, Prepositional, Accusative, Genitive, Dative, Instrumental.