What version of weblogic are you asking about?
Lots of great documentation at
http://e-docs.bea.com Assuming you've downloaded a newer version, you need to run the configuration wizard to create a domain. If you create a basic weblogic server domain you get to choose "Express" or "Custom" creation. With custom you pick the port, with express it uses the default which is 7001 with the versions I've used. Not to worry, though, as the server prints out the port number when it starts up. ("listening on port 7001")
When you create the domain, if you look closely you'll see where it is going to create the domain. It'll be like "bea_home"/user_projects/domains/mydomain.
If on windows you can start from start - all programs, or find the start script in you domain directory.
Inside the domain directory there's an applications directory. That's where you can drop a web app, etc.