To be honest with you, since you have another device doing NAT, you are mostly protected already. A firewall will do very little. The whole point of using a firewall, is to have a public address under your control, that ability you will loose when using instead the cheap ISP router doing NAT.
Secondly, it is recommended that Cisco products are installed and configured by reputable consultant or certified Cisco partner. If you want to do it yourself, you will have to start from the public documentation ad many examples, all freely available on Cisco.com, but be prepared to spend a long time dealing with things that may be osbsucre or with no meanig to you.
