I'm a legal resident with an expired card I want to travel to Mexico my question is I want to be able to comeback legally?

If your green card expired, it does not mean you have lost your status. It does mean that you need to file I90 and get the card renewed before you can travel. If the "expiration" refers to the two year green card, you need to talk to an immigration lawyer as this is a much more complicated issue.   Best Immigration Lawyers.