I came to US just after being 26 so i was exempt from selective service ,Now i'm filling out FAFSA , as I 'm exempt from selective service ,how do I get the proof that I was exempted ?Plz share your experiences.

You can show ur passport stamp explaining you were 26 at the time you came to US. Your financial adviser should be able to help you.

Non-U.S. male who came into this country for the first time after his 26th birthday
Date of entry stamp in his passport, I-94 with date of entry stamp on it, or a letter from the U.S. Citizenship and Immigration Services (USCIS) indicating the date the man entered the United States. If the men entered the U.S. illegally after his 26th birthday, he must provide proof that he was not living in the U.S. from age 18 through 25. Resident Alien Card (Green Card) is not valid as proof of the date of entry to the United States.

Non-U.S. male on a valid non-immigrant visa 
For example, if the man entered the United States as an F-1 student visa and remained in that status until his 26th birthday, he would need to provide documentation indicating that he was admitted on an F-1 visa and attended school full-time as required. Acceptable documentation for this situation include a copy of his I-20 form or a letter from the school he attended indicating his full-time attendance as a non-immigrant alien. The same thing applies for all non-immigrant statuses.

As far as I remember correctly in the same situation, I wrote/applied to the selective service about registering, and Selective Service sent me a later stating that I do not need to register with them due to my age. So I submitted a copy of that letter to FAFSA and worked well. My advice is to contact Selective service via any means.

Himal's suggestion will not work even though his points are valid.

For you to complete FAFSA, you need to directly request them a selective service status letter.
Check this link out:
https://www.sss.gov/Portals/0/PDFs/Status.pdf
Print it out, fill it up and mail it to :
===========================
Selective Service System
ATTN: SIL
PO Box 94638
Palatine, IL 60094-4638
===========================
Once you send them a request mail, they'll send you a letter saying you are not required to register for selective service. It takes about 3-4 weeks to get that letter. Hope that helps you.