I am new to ASA firewall. I have ASA 5520 firewall (hardware) I want my clients in my Local network to gain internet access through the firewall. So it should be like this: ISP to Billion modem to Firewall (hardware) to Switch to client.
though your question is really vague I think you should do source NAT for internet access for your clients (aka MASQUERADE)
imagine your client network 192.168.1.0 with a subnet mask of 255.255.255.0
you should config this with CLI (supposing you have configured everthing else such as interfaces and security-levels etc) :
object network myclients subnet 192.168.1.0 255.255.255.0 nat (inside,outside) dynamic interface