Yeah - basic home-networking is typically pretty straight-forward. You'll want to figure out your basic services (DHCP, DNS, and routing) but after that it's pretty simple. OpenWRT should handle the DHCP and routing. I'm not sure about DNS though.
DHCP will tell systems "here is your IP, here is the CIDR of the network you are on, here is the router that handles traffic for things NOT on that network (e.g. the internet), and here are the DNS servers you should use for name resolution.
With DHCP you can also hand out "static leases" to give systems reliable IP addresses based on their MAC addresses. Then you can setup a DNS server that does internal name resolution if you want to be able to reference systems by name. This DNS server doesn't need to be publicly available (and indeed should not be).
The Firewall is typically only for things coming into your network from the internet. You can restrict outbound traffic as well if you want but that's less common. By default things on the internet will NOT be able to get to your internal systems because of NAT. So to allow things "out there" to access a service running on an internal system you'll need to do port forwarding on your firewall. This will a) open a port on the internet side and b) send all traffic to that port to a port on an internal system. The router will handle all of the network-to-network and traffic handling stuff.