How do I complete the effective date of payment and certification information on a DD Form 108?

Hello everyone. Here is my situation. I was in the Navy and I served 11 yrs on active duty and 14 years in the reserves. I receive a disability check from the time I was in the reserves to the present. I am ready to turn in my paper work to receive my retirement pay, age 591/2. From what I gather there are only two forms that I need to fill out, DD FORM 108 JUL 2002 and DD FORM 2656 APR 2009. DD FORM 108 I did not have a problem filling out. DD FORM 2656 is the one I am having trouble with. The sections are IV and XI. On section IV 12bthey want the effective date of payment, not sure what that date was or where I can find it. I almost think from question 12a, that it will be the date when I do retire with pay. I'm just not sure what date and where to find it. the other section is XI, certification. I have been out since 2013. does this need to be filled out or is it N/A?
So if there is a number that anybody knows of or an organization that I can contact that would be great. I have been looking on ine and have not been able to find anything that makes sense. I am in the Fort Worth TX area.
Thank you.