Just enter another persons apple ID in the "to" box and then type your message and click send
Imessage can have go over wifi for iPod touch
Yes
iMessage is on iOS 5 and the iPod touch first and second generations can't support it.
No, you can use iMessage on a 3rd and 4th gen iPod Touch running iOS 5.
For iMessage on iPod touch you don't get a number people will contact you using the email you used when you signed up.
Yep
Just use your apple accounts for app store.
You can only iMessage with someone else who has an iPhone, iTouch, or iPad.
Yes, if they are both running iOS 5 or higher, and iMessage is enabled
Yes. iMessage
If you have iMessage installed you can only iMessage other iDevices that has iMessage installed on them. If someone has an iPad 2 but not with 3G but has an iPhone, then the iMessage will go there iPhone.
On an ipod touch you can or an iphone but nothing else... YET! :)