It is still very unclear and very general.

We can not produce a learning course here at forum. You can find a textbook and follow it and if something is unclear in particular example, or in particular paragraph, ask here.

I assume that your native is English and the very concept of cases is new for you. What you need if you like to get it theoretically is a syntax analysis which is much like in English. That is you should be able to discriminate subjects, predicates, objects and attributives and their subordination. All subjects and objects (which are mainly nouns or personal pronouns) are divided into several groups according to the particular situation in the sentence and each should take corresponding case. All attributives should agree in case with corresponding object or subject. That is the base.