You can prepare your return with MFJ using TurboTax, Print the return, sign, date and add a request signed by both you and your spouse that you wish to be treated as a resident alien for the whole of 2018 -- mail the whole package to the IRS.
Generally, F-1 during OPT are exempt from paying the FICA but because you are changing your status, you would be required to pay for FICA --- but generally you do not go back and pay the FICA -- you should be OK ----
Namaste
