说=say/speak , most common using in daily conversations. 我說我要一個麵包 ,我說我明天要考試 说话=talk, usually use it as a noun , 我聽到他說話 , 他正在說話 讲=say/speak , usually refers to a speech 我講話你要聽 你媽媽說/講的話你要聽 谈= usually use it as mention , 我談到我們下個月要出去旅行

Actually there's no essential difference among them. 说 is used as say. (他说……=he says…) 说话is a phrase. 说(verb)+话(noun) eg. 他没说话=he didn't say anything 讲 can be used as 说(他讲……=他说……=he says…) but 讲 have some other meanings: teaching or explaining. eg. 讲课=give a lesson. 讲解=explain (a question/rule) 谈 is a formal talk. eg.谈判=nigociate

说（shuo）、It means that express what you need in a sentence