Can someone tell me where to put my Mobil number for iMessage? I don't see a field except for email

You don't enter it. It is read from the SIM in your phone. If you want to use the iPad for iMessage, you have to log into the iPad with the same Apple ID you are using for iMessage on your phone. You also have to have wi-fi and Bluetooth turned on.

Try this support article > iOS and OS X: Link your phone number and Apple ID for use with FaceTime and iMessage