Wait for your EAD card to be mailed out. Once you have your EAD Card then go to SSA with your passport and a filled out SS5 form you can find at the social security website. If you don't have a passport you can use your birth certificate. Tell them you're applying for a social. If you're lucky you wait 2 weeks for them to mail the number to you. If not it'll be a bit longer but you'll still get it. Once you have your number take it with you to the DMV along with your uscis letters to prove residency. I think take your passport too. From there it should be simple.