it depends on which system u have. if u have xbox then u will need ethernet and highspeed and the correct internet provider. then hook one end to ur system and ur comptuer. the game will show up on ur tv(i think u can use ur computer too i duno).

when u put an online game disc in ur system then go to online menu and sign on or w/e then ur done.

for ps2 same thing but some games u will need highspeed internet to play online since some games wont let u use dialup. btw, xbox dont support dial up only highspeed. i duno about gamecube though. i feel like gamecube doesnt even have a online game since i never seen much online games for it. k bye now.
oh, u can : download,record ur name on top list or w/e, challenge other online people, talk trash, what else...? for ps2 same but u can surf the web too and u will need something for it i duno .