how do i file for green card for my husband he is in Mexico i am a US citizen.

It is ver important that your husband does NOT re-enter the United States illegally. An illegal re-entry after accruing years of unlawful presence will cause irreparable harm to his ability to immigrate. To start the process, you will need to file a Form I-130 on behalf of your husband. Once the petition is approved, your husband will complete an immigrant visa application. He will be interviewed at a U.S. Embassy or Consulate regarding the application at which time his application will be denied due to unlawful presence and he will be asked to file a waiver. You can start preparing this waiver in advance of the interview, but it cannot be filed until the consular office determines a waiver is necessary. This waiver is based upon extreme hardship to you if your husband is not allowed to immigrate. USCIS will look at both the hardship you would suffer if you live apart and if you went to Mexico to live with him. This is a complicated matter. I encourage you to work with an experienced immigration attorney. You can click on family-based immigration to learn more about the process.