Most likely you have a Shimano-style cartridge bottom bracket. Generally, if you think that the bearings need replacing it's new cartridge bottom bracket time. They come in different spindle lengths and crank interfaces so be sure you get one that matches.

It requires a special tool of course. Assuming you have an English threaded bottom bracket the drive side has a left hand thread. Sometimes they're a little tough to break loose. Otherwise it's a pretty routine job.