New Install ASA 5510

I have a router/modem connected to a switch and then to Lan..I'm trying to connect outside interface of asa 5510 to lan port of router/modem however no sucess. Router/modem uses dhcp to give ip address to lan and the router modem has it's own private and public address..I am giving the a private address to outside interface of asa 5510 same range as router/modem..doesn't work any help please..

I think we need more information. Can you post your configuration?

Can you do a show run along with a drawing of your setup?