Do I have to be single to petition for my dad and if I don't, can I petition for both of them at the same time?

Generally, a U.S. citizen 21 years of age or older may petition for a parent to obtain immigration benefits, and generally, a U.S. citizen who becomes married may petition for a spouse even if the U.S. citizen spouse has not yet reached age 21. Additionally, the U.S. citizen may file applications for both a parent and a spouse at the same time. In connection with two applications, if the U.S. citizen's income is not sufficient to meet the Guidelines requirements for the applicable family size, it may be necessary to have a "joint sponsor" in the application process. A joint sponsor generally may be any U.S. citizen or Permanent Resident who has sufficient documentable income and who is willing to assume the responsibilities associated with the Affidavit of Support forms. It would be wise for you and your father, and also you and your fiance, to consult with an immigration attorney who, after learning all of the relevant details about your family members, would be able to advise about eligibilities, options, and strategies, and would be able to offer legal representation in the often quite complex application process. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.