I would used the Database Configuration Assistant tool if it is installed. This is the easiest way to setup a database, I think. You don't have as much control as you would if you created it "by hand" (using SQL statements), but you don't need to know as much either. It pretty much steps you through the process.
To find the assistant, look at your Start Menu and find an "Oracle" entry. Under that there should be a "Database Administration" entry and under there should be the "Database Configuration Assistant". If you can't find it, you probably didn't install it.
By the way, if you installed Oracle Enterprise Edition or Oracle Standard Edition and it is properly licensed, you probably spent a lot more money than you needed to. You could use Personal Oracle (cheaper, and it some cases free - read the license agreement) as it has basically all the features of Oracle Enterprise Edition, but is only licensed for use on a local machine.