Modem and router sharing IP address

hi,  just stumbled upon you in a google search to reach d-link customer service...this is really cool!  hopefully you can help... i've had this problem a couple times before, but forgot how to fix it.  i just got a new computer, so i need to reconfigure my wireless set up so that the modem and router use different IP addresses.  i believe at the moment they are sharing the same 192.16.0.1, so that the wireless connection is dodgy and breaks after about 20 mins or so.   that said, it may break before i get your advice on how to fix problem.  but i'm hoping for the best. i'm using a MacBook Pro with a dsl speedstream modem and dlink router. thanks!! sarah