The big green app is on my iPad and iPhone...I don’t now how send texts on it ive got couple texts on it.. i ca Tex to those two...how do I text other people? thank you!

SOURCE: "error sending message" in my sms text message

This seems to be a recurring problem that I have discovered was a problem with both the original iPhone as well as the iPhone 3g. I just had this happen to me 3 days into owning the phone and the "solution" set forth to me by the apple tech at AT&T was to go into settings and place the phone on airplane mode for a couple minutes and switch it back. This surprisingly worked for me albeit for a matter of minutes. I was then instructed to do a Network reset also through settings under the "Reset" category at the bottom. Although it didn't immediately resolve the problem, after I manually entered my friend's number each time I texted for about 4 texts I have been able to text uninterrupted so far. Not sure this is a permanent fix yet but it's worth a shot for you guys as well.

SOURCE: text messaging not working iphone 3g

it might be a software trouble do resetting the setttings the the phone wil be as new

SOURCE: Getting an error when texting an iPhone

But it happens when i try to send a normal text message, not just when I try to send media messages.
Also, eventually, the message will send, it just takes a while. Sometimes I get the error up to 5 times before the message sends.

SOURCE: my iphone wont make calls, send texts & the 3G isnt working, help

check ur phone settings if your firewall state is on it should be off click call firewall state off,sms firewall state off.try n let me know

SOURCE: problems transfering apps from iphone to Ipade

You are able to use any appstore app with the iPad. Make sure your iPad is listed as a connected device in the left side bar of iTunes. Check your sync settings to make sure they're set to sync with your iPad and if needed restore your iPad using the restore button and try to sync again.

To make an application fullscreeen tap the small "2x" button in the bottom corner of your iPad. This will upscale the app to fullscreen.

Hope this helps, if you found this helpful please leave some feedback. Thanks.