Are you asking to get a timestamp with every single text message? That is unfortunately not up to us. There are jailbreak tweaks to achieve that, like this one, but with stock iOS, you are not always going to get a timestamp. After 15 minutes or so have passed without a new text being sent or received into any particular conversation, the next text that is sent or received into that conversation will get a timestamp.
Are you asking about seeing the time when a message was read... i.e. the timestamp with the receipt? If that's the case, then, provided you already followed RoofMonkey's advice, you will see the "Read hh:mm" until a new day has started. After the message falls into the realm of "yesterday," it states, appropriately enough, "Read yesterday." After yet another day passes, the word "yesterday" turns into mm/dd/yy, and you will not see the time it was read.