Once you reach retirement eligibility (20 good years) they should send a letter (certified?) to your home of record. This will contain a form to allow you to select survivor benefits plan.
If you don't return the form in 60 days or so, they automatically enroll you in the most conservative (and costly to you) plan.
You have to do it at retirement eligibility because one of the options provides your spouse with an immediate benefit if you die before age 60...so you have to elect or decline that immediately.
I think you have another choice to make when you turn 60, but it's all explained in the letter.