To use OpenDNS, you need to have their DNS servers specified for each computer.
The simple way to do this is to ensure that the device that delivers IP addresses to computers appearing on your network (using something called DHCP) also tells those computers to use the OpenDNS DNS servers
The device providing DHCP is usually your home router - the thing that connects you to the Internet.
We would need to know the specific model to give specific instructions.
Most routers will have a DHCP page when you can set the above. Sometimes, though, you just get to set them on the main page of the router. The sign-up page for FamilyShield gives both specific and generic instructions for routers. You also need to tell OpenDNS the IP address that represents the "outside" view of your home network. You can see this by searching on Google for "whats my ip".
Note that the only difference between the standard free service and the FamilyShield one is that the later is pre-configured to filter out certain things.
Once you have done the above, you should restart the computers so that they pick up the new settings. Then you should test to check if the configuration is correct. This page tells you how.
The final step is to know whether or not your external IP address changes from time to time. You should check with your ISP. If it does, you will need to have a method of telling OpenDNS what the address is. The Knowledgebase contains the required information, search for "dynamic dns".