It's not really possible to create separate hostnames for different applications or different Tomcat installations on the same server.
The hostname is the name of the computer itself in the IP network, it is not something that is configured in Tomcat. The name "localhost" is a special hostname, which is hardwired to the IP address 127.0.0.1, which always means "this computer".
If you want your computer to have a different hostname on the network, then you must configure the DNS (Domain Name Service) server which translates hostnames to IP addresses, but this is usually not something you can do yourself (your Internet service provider or network administrator can do this for you).
If you want to use alternative hostnames locally only (only on the computer itself, not useable when you connect from another computer), then you can edit the file C:\Windows\System32\drivers\etc\hosts. Add a line like this, for example:
127.0.0.1mymachine
and then you can connect via
http://mymachine:8080 But note that you cannot use this to separate webapps or Tomcat installations with different names.