If you are using the iMessage app on your iPad, you need to use the same email address you use on your iPhone. But, be advised, that when people email your cell number on your iPhone, you will not get the message on your iPad. iPad is not a cell phone. And also know that iMessage on iPad is intended to only let you text with other iOS (iPhone, iPad, iPod Touch) devices.
If you want to be able to text to anyone from your contacts list (those for example who do not own an iOS device), there are free apps in the App Store, such as TextFree, which you can download. But, TextFree will assign you a number (I know kind of odd since iPad is not a cell phone), which you can then text anyone you want. You'll just need to let your friends know the texts are coming from you.