If you're stone obsessed, you buy a stone for the special sharpening or polishing qualities or aesthetics.
Otherwise, buy a darnn stone and try the darnnn thing out. Start with sub $200 Watanabe for gosh sakes.
These kinds of questions pop up due to the cost, and that's understandable so, for entry level:
(1)Don't buy a giant stone
(2)Don't buy a pretty pattern stone
(3)Don't buy a rare, stamped stone
(4)Don't buy a super in demand stone (cough, ohira renge, okuda suita)
(5)Don't buy a super hard fine stone
(6)Don't buy a high-end mine name stone
(1)Buy from a reputable dealer so you don't get sand in your stones, or stone that disintegrates (though this is never guaranteed, the reputable dealer will refund you and shipping)
(2)Buy a finishing stone as your first jnat
(3)Buy more direct if you want for possibly cheaper, more rare, or more bang for buck, but expect worse customer service usually. Retail is there for their help.
(1)Expect to spend at least around $100-$200
(2)It costs more to get nice rectangles. It's very helpful to have rectangles for bulk sharpening.
(3)Generic no name Japanese mine "honyama" is just fine.