How do I add a netmask?

Excuse me, as a novice in network configuration, I'm wondering how I can effectively add a netmask to my network settings. Could you please guide me through the process in a step-by-step manner? I've heard that netmask is crucial for defining the network address and the number of addresses available in a subnet, but I'm not sure how to incorporate it into my current setup. Would it involve editing the IP configuration on my router or computer? Any specific commands or tools I should be aware of? Your assistance in this matter would be greatly appreciated.