Q: How can I send and receive iMessages to non-Apple devices from iPad?

iMessage only works with iOS devices and Mac computers running

later versions of OS X. You can set iMessage to send as SMS to

not iMessage users if your wireless account is provisioned to allow SMS.

http://www.apple.com/ios/messages/

Your iPad is not a cellular telephone.  As a standalone device, the iPad has no capability to send or receive SMS text messages (to/from non-Apple devices).

However, if you have an iPhone, you can use the iPhone's Continuity feature to have your iPad connect to the iPhone to send/receive SMS messages.

See "SMS" here:

Use Continuity to connect your iPhone, iPad, iPod touch, and Mac - Apple Support