Q: please tell me how to start ipad cellular internet...

I'm not sure what you mean by "start", but perhaps these support articles will help:

http://support.apple.com/kb/HT4146

http://support.apple.com/kb/TS4249

This article shows how to connect to AT&T, probaby not directly applicable in your case, but does illustrate how you set up an account with many cell carriers.

http://support.apple.com/kb/HT4157

If those articles do not help, please post back and explain your situation or question in more detail.

Regards.