Will I be able to get a green card if I marry a US citizen but I have overstayed for 6 months?

Generally, if a foreign national enters the U.S. lawfully and with inspection (such as with a visitor's visa), and then becomes married to a U.S. citizen, the couple can succeed in a marriage-based adjustment of status application process in the Immediate Relative visa category (so the foreign national will become a Lawful Permanent Resident - get a "Green Card"). This is true even if the foreign national spouse may have overstayed an initial visa by six months. Of course, there are many other details that determine eligibility too. There really is no substitute for you and your fiance to consult with an immigration attorney who, after learning all of the relevant information, could advise about eligibilities, options and strategies to attain your goals, and who then could offer legal representation in the often complex application process.