Good day I want to apply for a b2 visitor's visa to go to USA from Zimbabwe, as my sister in law will be sponsoring the trip what do I need to do.

You don't need a sponsor for a tourist visa. Very few require that. Just go to the US consulate in Zimbabwe and apply for a tourist visa. You will have to show evidence that you have sufficient ties to your country that would incentivize your return home. That includes a job, real estate you own, family members etc. You also need to show why you're traveling here , how long you will stay and proof that you have sufficient resources. Don't say you plan to work or to even baby sit your brothers children. ( if any). USCIS will deny you.