One can import an OBJ file into free program Meshmixer, then export it to STL format. There are other programs that are free and paid (such as Fusion 360) that will perform similar tasks.
As noted by user agarza, Prusa Slicer will import an OBJ and will also export to STL format. PrusaSlicer is also free.
Basically all slicers work very similarly, it is a matter of preference, being accustomed, or wanting to use a certain (set of) features. Their job is to prepare the object to be sliced in layers to be executed by the printer you use. For every slicer to work properly, you need to configure the printer settings correctly.
Basically, all slicers have the ...