I see that you'd like to know how to open an SR, but I'd like to know what issue you are currently facing. All how-to questions, general questions and product enhancements can be asked here in the Product Support Community. We have a vast amount of people here who are able to assist you with those types of questions!
If you are experiencing a defect, then you can open an SR. Which can be done by hover over Support in the header, clicking on Pega Support and then on My Support Portal.
I tried above step but getting not authorized to create/modify error when i submit the SR . Do i need to get any privilage to create SRs from pega systems or Do i need to do any other things to create SR?