Only my principal should sign exam form??

If your articleship training is still continuing, in that case, you can get the signature only from your principal.

I did not complete my articles yet..thank you sir

Then in that case, signature on the examination application needs to be obtained only from your principal. 

Dear Lavanya,
On the Examination form Any member of ICAI

Or Gazzet officer can sign.It not specified that only practicing CA which u are gone into prctical Training.

Do i need to enclose certificate of service along with the print out of online application