I made a Minecraft server from the download on the Minecraft site, and inside of the server-properties file it says "Server-IP:" is this where I type a custom IP? Or what...? If you can help please do!
Since most home users have dynamic IPs (change at random times), it's better to use a domain name. You can get free ones from places like NoIP.com and create a domain name. This way, you don't have to keep giving out IP addresses to people when they change. Your computer runs an updater utility that keeps the IP synced with your domain name should it change.