Q: iPod Touch 1st generation-texting

There should be a built-in app called Messages.

I'm not sure; I have a 1st gen iPhone, but not iPod touch....  This article as information about Messages

http://en.wikipedia.org/wiki/Messages_(application)

The 1st gen iPod touch (like the 1st gen iPhone) can run up to iOS 3.1.3 (called "iPhone OS" back then), but not later iOS releases.  If that iPod touch you purchased has an earlier release of iOS installed, you can upgrade it

http://support.apple.com/kb/HT2052 (The upgrade costs $4.95.)

Since you already purchased the iPod touch, I would wait until you receive it, upgrade the OS if necessary, and then see what it can do with the built-in software.  You can install apps from the App Store, but it is somewhat difficult to finds apps that support the old hardware and iOS these days.

I have an app called VintApps that helps me find apps in the App Store that still work with my old iPhone.  It found these that appear to be for messaging

https://itunes.apple.com/us/app/bulksms-sms-text-messaging/id423765990?mt=8 (BulkSMS)

https://itunes.apple.com/us/app/hello!-ad-free-texting-messaging/id359216159?mt= 8 (Hello!)

I don't understand...  This seems to show someone installing iOS 3.1.3 (although not in the official "legal" way).

This document explains how to update iOS devices

http://support.apple.com/kb/HT4623

You can do it directly from the device itself (if it has iOS 5 or later currently installed).  You can also use iTunes to run the update.  From iTunes, you can also run a Restore, which erases the iPod and reinstalls its software.  If this is a used iPod touch, you may want to do a Restore to give it a "fresh" start.

It can run up to iOS 6.1.3, although some features (such as Siri and panorama camera mode) are not supported.

You may want to ask the question about texting with an iPod touch again.  You should do so in the iPod touch category, not the Older iPod category (the current category).

https://discussions.apple.com/community/ipod/ipod_touch