You can only use NEC PC-98 keyboards. Normal keyboards have PS/2 plugin. But NEC PC-98 keyboards have DIN8pin plugin. You can find adapter to convert PS/2 plugin to DIN8pin, but it costs more than a PC-98 keyboard, and the keys are not exactly the same as PC-98. I don't recommand it. You can find a good keyboard for less than 30 dollars on yahoo auctions.

Same as keyboards, PC-98 mouse is necessary to use on PC-98. Normal PS/2 mouse will not work. You can find two types for mouse. One is old type and it does'nt works with all games. It is a flat long type (seems like a 9 pin RGB plugin).  The other is quite similar to recent mouse and has a round 9 pin plugin like the keyboard. This is definitely the best choice for recent models of PC-9821.

MSX 2 buttons type joypads are easy to connect to PC-98, compatible with lot of games, are very comfortable, and not so expensive. You just have to connect the 9 pin RGB type plugin to the one behind the PC-98 (if there is none, you can use sounds card for this plugin)

4 buttons joypads are great for most complex games, like fighting games. But hard to find, expensive (around 60 dollars), and not so confortable. Because manufacturers were not as good the one that made MSX pads, the quality was under also.