Ancestry.com is a paid site, a subscription is required to access their database.
Unfortunately Ancestry.com do not provide BMD records of Mexico, unless that person was a citizen. But border crossings and some documents relating to entry into the U.S. are available. If your family traveled to the States, they would have had a card documenting their visits when crossing the border.
As Jon said, the familysearch.org is your best source for BMD records of Mexico. The older records are very helpful,
and may have information regarding a marriage, grandparents, or children.
Good luck in your search, Susan