What is the best way for a person from Mexico to get a visa to come to the United States?

The starting place would be to examine what visas you may be eligible for. Visas generally fall into two categories: those that are based upon a family relationship and those that are related to employment, and there are extensive details about eligibility for each type of visa. After examining eligibilities, the next step generally is to review the pros and cons of each option in order to select the option that best suits the goals of the foreign national; this includes such things as the time needed to achieve success, the costs involved with each option, the likelihood of success, etc. Some immigration law firms, including mine, offer legal services on a "flat fee" basis so that a client will know the total expense from the very beginning, and a few immigration law firms, including mine, offer an initial consultation free of charge.