How do I file a petition if I am an American citizen and I plan to marry a non-immigrant from Africa?

More information is needed to fully respond to your question, but generally a U.S. citizen who becomes married to a foreign national who entered the U.S. lawfully and with inspection, may sponsor the spouse through an Adjustment of Status application process. That includes a Petition for Relative Alien, an Application for Adjustment of Status (with Applications for an Employment Authorization Document and, if qualified, for Advance Parole), along with all required supporting documents. If the foreign national is located overseas, then the U.S. citizen may apply for a fiance visa, or may get married abroad and complete "consular processing." If the foreign national is in the U.S. but entered unlawfully and without inspection, further analysis must be made to determine eligibilities. There really is no substitute for engaging an immigration attorney who, after learning all of the relevant information about the foreign national spouse and about the relationship, could advise the couple about immigration eligibilities, options and strategies.

Is that non-immigrant in the US. Then you get married first, and then file i 130. If not, you marry and file i 130. If you first want the non-immigrant to come to the US, you file i 129F, but only if guys met in person within the past 2 years.