How can I help my husband get a US citizenship if he and I got married in Mexico, I am a US citizen and he has a visa?

You need to file I130 for him, if he is in the US, you can file the adjustment package at the same time.

Generally, a foreign national who entered the U.S. lawfully and with inspection (as it sounds like your husband did with his visa), and who is married to a U.S. citizen, may apply to adjust status to become a Lawful Permanent Resident (to get a "Green Card"). Approximately three years after becoming a Permanent Resident, the foreign national spouse will be able to apply to become a naturalized U.S. citizen. All of this is true regardless of whether the couple became married in the U.S. or in another country. There are many additional details that address eligibility for a marriage-based adjustment of status application (and for later naturalization eligibility), and it would be wise for you and your husband to consult with an immigration attorney. After learning all of the relevant information about you and your husband (including the type of visa he used to enter the U.S., the time between your marriage and his entry into the U.S. with that visa, and many more details), an attorney would be able to advise you about eligibilities, options and strategies and could offer legal representation in the adjustment of status application process - a process that often is significantly more complex than it first may appear.