To report your IP to dyndns, you don't need to do anything with iptables, just
The dyndns update client software, just needs outgoing access, so no firewall configuration should be necessary.
You would, however, have to set up port forwarding from your router, and depending on your LAN setup, need to set up port forwarding rules with iptables in order to allow access to services running on your computer, such as 80 for http, 21 for ftp, 22 for ssh...
2. Configure port forwarding and firewalls
Now that your service is installed and running, the next step is to configure your network to allow remote access. To do this, you will need to configure your router's port forwarding settings. Please visit PortForward.com for a list of guides by router and service. (For reference, any service you can access with a web browser uses HTTP on TCP port 80). You will also need to allow this port in your firewall software, such as Windows Firewall or Norton Internet Security.
As for configuring iptables itself: