If you are legally married as 12/31/2018 then you can only file the 2018 tax return as either Married Filing Jointly or Married Filing Separately. The best way to file when married is MFJ. You claim your child as your dependent on the MFJ tax return.
On your tax return you enter the name as shown on your current Social Security card. Your last name and your spouse's last name do not have to be the same on a tax return. The name on the W-2 and the name on the tax return also do not have to be the same, only the Social Security number must match.
