Are you referring to a static IP address you can use within your home network?
Or a static IP you can use to access OctoPrint over the internet from outside your home network?
Regardless of the above scenario, it will also be helpful to know what kind of router/gateway you are using on your home network.
It may be easier to just set up a DHCP reservation in your router rather than having you edit the Debian configuration files. Of course, that's up to you. If you're comfortable editing the configuration file, either method will work to establish a static IP within your local area home network.
IF you are trying to get a static Public IP address, things will be a little different especially if you are using a consumer level ISP (Non Business Class). Most consumer ISP's use DHCP to assign dynamic addresses to their cable modems / DSL modems, etc. If this is what you're wanting to do and you do not have a known static IP block issued by your ISP, youwill need to research something called Dynamic DNS providers. These will allow you to register a hostname with a DDNS provider and the service will update your DNS Host Record entry every time your dynamic IP changes, so all you need to remember is the hostname rather than an IP address. Depending on the type of router you have, this can be set on the router, or if your router doesn't support DDNS synchronization, you can install a service on your Pi that will handle the DDNS synchronization.
Sorry, that was long winded, just want to be clear on your intentions so we can guide you in the right direction!