How to fix my problem on my iPhone 3 (not 3G or 3GS, I mean 3)?

I have an iPhone 3 with the old messages app instead of the imessages app. I'm not getting a cell phone plan, mind you. I don't have one and I'm not getting one. My phone can't text using the messages app. How can I fix this?

At first, make sure your text messages option is on. Then restart the message App. If it still doesn't work, try restarting iPhone. Or re-install the messages App.
Check text messages: Settings>Messages>Text Message to check whether this is toggled to ON.

Lost messages in any situation, please refer to this post, it helped me once:

There's no iPhone 3.So it has to be 3G or 3GS.

Well of course the problem is no service plan, therefore that app, which can only send texts will not work.

You need to get a plan on it.