vivi:
The best way to know whether one needs to file a waiver is first to file I-130. Once I-130 is approved and visa becomes available and visa interview is set at the Embassy, the Embassy captures the applicant's biometrics and conducts extensive background check. This is where the inadmissibility comes up. The officer at the Embassy will explain to the applicant under what section of the INA they are unadmissible, what forms to file (I-601, or 612). Note applicants who were deported need both forms, but applicants who never come to the usa need only I-601. You cannot file waiver unless you have an interview and found inadmissible. If the applicant is here in the usa, different process applies...
Good luck