Immigration law is complicated, and mistakes can follow you for years. Getting advice from the right person is one of the most important things you can do.
Only two kinds of people may give you legal advice on immigration:
An attorney (lawyer) who is a member in good standing of the bar of a US state or territory.
A DOJ-accredited representative who works for a nonprofit organization recognized by the Department of Justice. These organizations often charge little or nothing.
Others (friends, travel agents, tax preparers, "immigration consultants", people who say they "did it before") may only fill in forms for you exactly as you say, for a small fee. They may not tell you which form to file or what to answer.
In many countries a notario público is a lawyer. In the US, a notary public is not. A notary only witnesses signatures. People who call themselves "notario" or "immigration consultant" and give advice are breaking the law. Mistakes they make can hurt your case for years.
Recognized organizations and accredited representatives: search "Recognition and Accreditation Roster" on justice.gov (EOIR). It lists them by state.
Free lawyers: search "EOIR pro bono list" for free legal service providers in your state.
Attorneys: check the lawyer's license on the state bar website. Many state bars run a lawyer referral service, often with a low-cost first meeting.
Law school clinics: law students supervised by professors, often free.
Universities and employers: your international office can explain general rules and refer you. For work visas, remember that your employer's lawyer represents the employer, not you.
Refugees: your resettlement agency helps with many forms.
Questions to ask before paying
"Are you an attorney or an accredited representative? In which state?" Ask for the bar number or the DOJ listing.
"What exactly will you do, and what is the total fee?" Get it in writing.
"Will I get copies of everything you file?"
Bring your passport, I-94, every USCIS letter and any court papers to the first meeting, with your questions written down.
Promises of a "guaranteed" green card or work permit, or "a friend at USCIS".
Asking you to sign blank forms, forms with false information, or forms you have not read.
Charging for blank government forms. USCIS forms are free at uscis.gov.
Keeping your original documents.
Payment only in cash, or no written contract or receipt.
Calls or messages "from USCIS" asking for payment by gift card or wire. USCIS never does this.
Report it to your state attorney general and to the Federal Trade Commission at reportfraud.ftc.gov. Many states have special laws against notario fraud.
Choosing the cheapest "helper" and paying much more later to fix the damage.
Letting someone file a form for you without reading it. You are responsible for what it says.