Categorically - you are protected by a default rule set when you are activating. You will still need to fine tune the settings, but you’re covered.
When first setting the firewall up, if you run a LAN it is important to go through the Zone Wizard and then add the defined Zone as a trusted zone. This ensures LAN traffic is allowable to and from the PC with CPF (Comodo Personal Firewall) installed.
Have a browse through the FAQs. These have been built by the users on the forum and are based around real world issues they found with CPF and the solutions.