How to activate your iMessage

Use FaceTime with your iPhone, iPad, or iPod touch - Apple Support

What happens when you try? Error message(s)?

Messages - If you get an error when trying to activate iMessage or FaceTime.    

Messages Troubleshooting.   

Some other things you can try. Do you have access to another WiFi network where you can try? That has worked for several people.

Try a restart.

Restart your iPhone, iPad, or iPod touch         

Try a forced restart.

Reset Device.      

If necessary

Hard reset.        

Do a backup and try a factory reset. Then restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.