If we get married before she apply, will I receive a green card when she becomes a citizen?

If you entered the country legally, you will be able to adjust once your wife becomes a US citizen. Otherwise, you will need a waiver unless you can show you were paroled into the US.

A U.S. Lawful Permanent Resident ("Green Card" holder) may petition for a spouse in the Family-sponsored Second Preference (F2A) visa category. A U.S. citizen may petition for a spouse in the Immediate Relative visa category. The foreign national spouse generally must be in lawful status in order to be eligible. A spouse of a U.S. citizen will not automatically become a Permanent Resident (get a "Green Card") when the Permanent Resident spouse naturalizes to become a U.S. citizen; instead, it will be necessary to go through a marriage-based application process. In some instances it is not necessary for the foreign national spouse to be in lawful status in order to be eligible. There are many additional considerations that address eligibilities and options for a couple like you and your fiancee. It would be wise to work with an immigration attorney who, after learning all of the relevant details about you, could advise you and your fiancee about eligibilities, options and strategies and who could offer legal representation for the 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.