Doesnt your ISP provide you with a modem? all the ones in my area do (you probably dont need to buy them, call to conferm)
basicly the best way is to check the dell site (it should show you your machines specs). If its a laptop it probably has one built in, but again conferm. If its a pc then check the back to see if theres a little antenna. I hear some on mother boards have it on board but I've never seen one so I dont know.