What paperwork do I need to file in order to fix my mom’s papers if I am 21, a US citizen and my mom is in Mexico?

You need to start with I 130 form.

Generally, a U.S. citizen may apply for a parent through filing a Form I-130 Petition for Alien Relative plus (1) "consular processing" if the parent is located abroad, or (2) an application for adjustment of status if the parent is located in the U.S. The documentation to be filed with these applications includes biographical information forms, an Affidavit of Support, birth records and other supporting documents. Beyond the parent-child relationship, there are many other details that will determine eligibility. It would be wise for you and your mother to consult with an immigration attorney before initiating application steps; after learning all of the relevant information about your mother, including immigration-related history, the attorney would be able to advise you about eligibilities, options and strategies and offer legal representation for a process that often is more complex than it might appear.