What can I do for her to get green card?

You cannot really help her until you become a citizen: only a marriage to a citizen would excuse her illegal presence in the U.S. after expiration of her visa. If you marry her now, you might consider filing an immigrant petition for her right away - it will shorten the process by a couple of months, and you will pay the current filing fee (which is likely to go up before you become a citizen). More importantly, it will give your fiancee some degree of protection from deportation: if Immigration & Customs Enforcement ever becomes aware of her illegal presence, as a wife of a permanent resident with an approved immigrant petition, she will less likely be placed in removal proceedings, and if it happens, the Immigration Court will look at the case a bit differently and consider the hardship the deportation would inflict on you (and on your children if you will have them by that time). If you decide to follow this suggestion, remember that: a) the filing of a Form I-130 petition or even its approval do not give your wife any legal status or rights in the U.S.; b) even with an approved I-130 petition, she can be deported; c) after you receive a notice that I-130 Immigrant Petition was approved, do not file an adjustment of status application Form I-485 - it will be denied, and your wife will be placed in removal proceedings; at that point, ask an immigration attorney to handle the case with the National Visa Center to prevent it being discarded as abandoned.