Sir,
You are supplying T-Shirts, note book and pen to those who attend the seminar. I hope it is free of cost. If you include the cost in the fee, if any, you collect, then when paying GST on the event management service you can adjust the credit of GST paid on these articles and adjust the credit against payment of GST on the event management service provided by you.
If you are not charging any fee from the participants, then the articles supplied by you are free of cost and the transaction value will be zero. Hence GST payable will also be zero.