First, Where are you located and what country did you buy your idevice? Some countries have restricted facetime, ie middle east, UAE.
Check if you signed in Facetime with your Apple ID and password (Settings>>Facetime).
Check if you have signed iCloud in.
Check that FaceTime is not disabled in general/restrictions. If that isn't the problem try changing dns servers on your iPad.
Go to Contacts app and select the person you want call then tap Facetime.
If not works, check if the person who you are trying to Facetime has signed in his/her Facetime too.
Most of times the Facetime calls work calling from your friend's email address instead of mobile number. So try to call your friend both ways (his/her mobile number and his/her email address).
If still not working:
1. Go to the Settings App.
2. Tap on Wi-Fi
3. Your chosen network will be ticked. Tap on its white arrow over a blue circle button to its right
4. Tap on DNS
5. Replace the current information with 184.108.40.206,220.127.116.11
6. Try visiting a website with Safari to see if this works
7. Try registering iMessage / FaceTime
If still not working, go to Settings -> General -> Date & Time -> Set Automatically: ON (Yes date time setting has been known to cause issue).
Hope it will be helpful
Using FaceTime http://support.apple.com/kb/ht4319
Troubleshooting FaceTime http://support.apple.com/kb/TS3367
The Complete Guide to FaceTime + iMessage: Setup, Use, and Troubleshooting
Troubleshooting FaceTime and iMessage activation
Using FaceTime and iMessage behind a firewall
iOS: About Messages
Set up iMessage
Setting Up Multiple iOS Devices for iMessage and Facetime
FaceTime and iMessage not accepting Apple ID password
Unable to use FaceTime and iMessage with my apple ID
For non-Apple devices, check out the TextFree app https://itunes.apple.com/us/app/text-free-textfree-sms-real/id399355755?mt=8