There are multiple, legal steps for Mexicans to take to become an United States’ citizen.

The general overview of steps are:

1. See if you’re eligible.
2. Overcome illegibility.
3. File USCIS Form N-400
4. Get Fingerprinted
5. Attend a Citizenship Interview
6. Attend an oath ceremony

http://www.alllaw.com/articles/nolo/us-immigration/steps-become-american-citizen.html