Reported speech is differs from English to Russian. In Russian, the reported speech remains in the tense in which it was spoken originally. So, if you want to translate the sentence: 'I said that I would cook dinner tonight,' you would find out in which tense it was originally spoken (ie. I will cook dinner tonight - future) and use that for the reported speech. Thus the translation would be: 'Я сказал, что я приготовлю ужин сегодня вечером.' This clearly differs from English where we would simply use the conditional in this instance.