you need to prove that you have at least one jewish grandparent from either your mother's or father's side of your family. This is the Israeli law of return. You will need to show documentation- ketubah-marriage contract, circumcission-brit milah papers and/or gravestones. If you don't have these, then you will need to convert overseas, it can be a Reform, Conservative or Orthodox conversion and be a part of a Jewish community overseas for at least one year. If you convert with an Orthodox conversion, you will be allowed to marry in Israel. Reform and Orthodox conversions are permissible for immigration/aliyah and for saying "Jew" on your teudat zehut/identity card, but not for marriage in Israel.