If you don't understand when and how the endings change, you need to learn about the Russia case system. Any decent Russian textbook will explain it, or a teacher can help you. Of course both is best :) It's quite complicated so you need to slowly study each case in turn, don't try to cover them all at once as you will get confused. Practice the endings by repeating sample sentences (flashcards are great for this).

Russian language is different from the Indo-European languages in structure. We don't have strict word order, but we have cases, which have simplified and replaced this system. For a start I would recommend to find a teacher of Russian language. It will help to deal with conjugations and declensions. Then you can continue without a teacher. I will say from my own experience A teacher motivates you, your progress with the teacher will be much quickly and correctly.

Thank you for your advices, and I'm sorry to late my reply. Finally, I found a teacher who lectured me Russian. I'm looking forward to taking conversation by Russian!

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.

Hello Takafumi, how did you studied Japanese? Did you ask yourself "How should I learn Japanese?" I think no. :-) Try the same method with russian :-) Listen a lot of russian video or speech first. And slowly you will be able to understand more and more. The first thing every child do while learning language is listening. The grammar and reading are the next step.