Q: how can i txt sms on ipad mini ios 8.1

Your iPad is not a cellular telephone.  This is true whether it has cellular capability or not.  The iPad cannot send or receive SMS by itself.

However, if you have an iPhone, your iPad may be able to send/receive SMS using that iPhone.  See the "SMS" section here:

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