Router Connect to T1 Shared by Mult. Co.

Need to connect to a T1 shared by multiple companies within a single building. My understanding is that there is no hardware in place at the T1 to segregate the different companies within the building, and we want our LAN to be separate and secure. Was told by building IT that a router with DHCP enabled was necessary, but was not given any router settings.

Have a Linksys router in place; PCs (WinXP Pro) can see each other, but have no internet access. Played with the router settings a bit without success. Would appreciate any suggestions – thanks!